Marina revealing her VIH disease to the entire class. Because of this, Nadia starts giving her the cold shoulder due to her religious beliefs she gets from her family. It doesn't help that Guzmán calls her out on it in defense of his sister.
Guzmán attempting to commit suicide out of guilt for Marina's death. Lu manages to talk him down, to the point that she tearfully threatens to kill him herself if he tries that again.
Every scene involving Yusef yelling at his children Omar and Nadia, especially when Azucena reveals Omar's sexual orientation and Yusef confronts Omar, angrily asking to tell him it's not true, which Omar falsely denies.
Omar's fight with Yusef as he rants about how he's always kept inside the house against his will and not allowed to do whatever he feels like. Omar also attacks his father and shoves him to the ground, which results in Yusef furiously kicking him out.
Yusef forcefully dragging Nadia out of the school when Azucena attempts to change his decision about making her drop out. Guzman jumps in and tells Yusef not to treat Nadia like that, but Yusef angrily strikes back, giving him a Big "SHUT UP!".
Marina's death. She is all alone by the pool, when Polo shows up. All he wanted from her was the watch - to protect Carla's family and at the same time try and win her back - but when she repeatedly refused and harshly pointed out how submissive Polo was to everyone around him, he got impulsive and used the trophy to hit her in the skull. Polo's face quickly changes to a shocked one. He definitely feels no pleasure at suddenly killing his former friend and his best friend's sister. Marina's reaction is also as heartbreaking.
On the first day back to school since Marina's death, Samuel arrives and the first thing Guzmán does is to ask what he's doing there, then furiously attacking him before Azucena breaks off the fight. Despite Azucena trying to reason with Guzmán that Samuel is not to blame for Marina's death, however, Guzmán is still grief-stricken and blinded by his hatred of the poor guy to even care and makes it clear he doesn't want to see or talk to him at all. He also threatens that anyone who he sees talking to "the waiter" (the Embarrassing Nickname he gives to Samuel) will be dead to him. "It's either him or me", he adds.
Christian being ran over and hospitalized by Carla's father. As far as we knew, Christian never signed up for teaming up with Carla and Polo to cover up Marina's death, so he tried to right the wrong himself for once, only for this incident to blow up in his face.
A pretty minor one, but Nano's clear concern for Samuel once he gets out on bail. He's immediately suspicious of not only how Samuel came to make enough money to afford his bail so quickly (not knowing it was really an annonymous donation from Ander and Polo), but also of how cagey and aloof he's being when asked about it. It leads to him figuring out that he's begun working with Rebeka's mother as a drug runner, and he sternly warns him to quit, as it's what led Nano to become the career criminal that he is presently. Samuel winds up dismissing him due to the family's precarious economic situation.
"Now that I'm back, you're going to leave it."
Guzmán breaking up with Lu when he admits to cheating with Nadia. Lu's reaction is very heartbreaking.
Shortly after that, Guzmán shows up at the store begging Nadia to give him a chance at a relationship, and even revealing he broke up with Lu. However, a teary-eyed Nadia has to refuse because the last thing she needs is to be a disappointment to her parents, especially her father, who is obviosuly prejudiced against Guzmán. The way she delivers this line is really painful.
"Lu may be used to getting her heart broken...but my parents don't".
Lu's speech at the fundraising party expressing her anger at the people who have been lying to her face (i.e. Guzman's cheating with Nadia, exposing Cayetana as a congirl and the cleaner's daughter, as well as Carla avoiding her and her affair with Samuel) and how they always take her for granted and fail to acknoweldge all the good things she's done for them.
Lu posting the video to the entire school of Nadia and Guzmán having sex in the changing rooms to get even with her. Most of the students laugh at her.
Carla is forced to keep distance from Samuel as her father had threatened to harm should he ever see her with him, so she masks a frosty attitude towards him as a mean to keep him safe.
Pilar and Nano's big fight before Nano flees to Morocco. Nano's not only suffering from being framed for murder, but he's also going stir crazy over being forced to stay home, is worried about Samuel's involvement with Rebeka's mother's drug business and is trying to protect him, and has given up any hope of being found innocent. Pilar, on the other hand, is struggling not only to keep Nano out of trouble now that he's out on bail, but also feels Samuel pulling away from her after they both worked hard to raise money for Nano's legal fees. The blow up between them causes her to accuse him of being ungrateful towards her and Samuel's hard work for his sake and scathingly tell him that their lives would be better if he just left. Coupled with Samuel's disappearance soon after Nano goes through with his plan, it's no wonder Pilar spends most of her scenes set in the present day in a Heroic BSoD.
Valerio telling Lu that he cannot go back to Chile, because he tried to call his mother who hung up on him - while calling her from a different number - after he greeted her saying "Merry Christmas". Lu takes pity on Valerio and offers to let him crash at her place for a few days while her father is away on business.
Ander informing his classmates about his cancer. Lu and Carla's reactions to hearing their friend is sick are pretty serious and concerning.
Rebeka being broke because of her mother's arrest, which Samuel had caused.
Carla finding out that her mother knew about her father's plan to force her in a relationship with Yeray, whose money they could use to prevent the wineries from going bankrupt, even though she's not romantically interested in him. Carla is clearly heartbroken and feels betrayed at the fact the two people she cared about the most would willingly exploit her. When their wealthy lifestyle was being threatened, they were willing to do whatever it takes to ensure it remains the way it is. Carla's mother showing No Sympathy and telling her to "think about their future" was just outright cruel.
On Samuel's side, he only agreed to go through with the plan to get his family back, and is rightfully furious to find out that he had been tricked into giving up Nano's location, a decision he made in vain. He even lashes out at the Inspector, blaming her for not having done her job properly, despite trying to assure him that she was tricked too. Not only had Samuel lost his scholarship and chance of graduation, but he had also lost his family (probably for good this time), so it's hard not to feel bad for him when he feels his whole world came crashing down all in one day.
A minor one at the same time, Nadia deduces that Malick cheated on her with Omar after seeing them both covered in fluorescent paint. In response, Nadia wordlessly slaps Malick and leaves. Malick goes after her.
Polo's death in Season 3. Where do we start? This poor guy was never a bad person. He simply had problems with anxiety and panic attacks. He got dumped by Carla, bullied mercilessly after the truth about Marina's murder came out, belittled by his mothers, and blamed for things that were actually his mothers' fault (the expulsion of Samuel and Guzmán, and revoking Nadia and Lu's scholarships). Just as he was about to turn himself in and atone for what he has done for once, this is frustrated by Lu who accidentally impales Polo with a broken neckbottle. Lu immediately realizes what she's done and apologizes to Polo, saying she didn't mean to. Polo's response is that he knows, as he did exactly the same thing with Marina when she provoked him. He tries to approach Guzmán for help, but he bumps into some people on the dancefloor and falls off a second-floor glass balcony. Guzmán is the first one to yell out his name and rush to check on his friend. As Polo succumbs to his injuries, Guzmán tells Polo he forgives him and a weakened Polo hold his hand before he fades away.
Even worse, Polo only showed up to the graduation party to inform Lu and Nadia that their mothers would still fund their scholarships to Columbia after managing to convince them, only for everyone to think he's here to cause trouble and Lu to ignore him multiple times when he tries to tell her. Just as everyone is about to go through with the plan to frame Polo's death as a suicide, Guzmán begs Nadia not to parttake as it would cost her the scholarship. When Lu learns about the scholarship being reinstated, she genuinely feels guilty for not listening to Polo for once.
Cayetana saying goodbye to Polo's dead body and professing that she did truly love him, all while Rebeka distracts the guard to buy her enough time to put her fingerprints on the neckbottle and make the death look like a suicide. Cayetana and Rebeka also break down in tears and comfort each other in the aftermath.
Samuel and Guzmán's physical altercation at the end of Season 4. These two guys who initially started off as enemies and later became best friends, to again become enemies, have been wasting their energies fighting over Ari, a girl they both like. To see them fight like this after everything they have went through in Season 3 is just utterly depressing to see.
It even becomes sadder and Harsher in Hindsight if you rewatch Season 2, as Guzman's first instinct to seeing Samuel back at the school since Marina's murder was to beating him up and making it damn clear he's not welcome.
At the very same time while this is happening, Armando attacks Ari at the dock when she tries to tell her father that he sexually abused Mencia, leaving her to die. Ari tries to stand up but she falls into the lake. She gets hospitalized and fortuntately pulls through.
The reactions of her father and siblings when they arrive at the scene are heartbreaking. They try to reach her but the paramedics keep pulling them back.
Samuel's death in Season 5. He is killed by the principal Benjamin, whom he saw as a father to him. Samuel had in his possession a SIM card that contained solid proof of crimes he's comitted, and he tried to bribe Samuel into a silence with money and a college in Oxford. Despite this, Samuel chose to remain loyal to his values and principles, yet this ended up costing him his own life. Rebeka and Omar tend to and mourn their deceased friend. She even punches Benjamin in retaliation for what he's done, before the two walk away from the scene.
While Rebeka and Omar try to help Samuel, Ari tries to stay by Samuel's side, but Omar angrily rebuffs her, expressing his anger at her because of how he clearly never liked Benjamin since day one.
One can also imagine how his mother, Nano, Guzmán, Ander, Nadia, Christian and Carla would react if they learned about Samuel's death. The fact that all of them left before this even happened, is what makes it all the more tragic as they Never Got to Say Goodbye.
Carla is the most tragic example, as she had lost her first boyfriend Polo, and now she's lost the love of her life for good.
Isadora getting raped by Hugo, Alex and Javier, and them getting away with it, by making her look like an Attention Whore. Because of this, she is ostracized by most of her classmates.
Benjamín's fight with Patrick and him slapping his son.
Cruz's death. Iván's reaction upon finding his dead boody, with his face soaked in blood.
Dídac being judged by most of his classmates for his family's supposed involvement in the shooting. The parents demanding his expulsion and the students making over-the-top death threats to him only makes it worse.
Omar expelling Joel from his flat after he caught him and Iván having sex in his bed. Iván stays to comfort Omar, but Omar tells him to leave as well, only for Iván to resist and request that he hit him. Omar keeps telling him to leave while hitting him until he stops and breaks down in Iván's arms.
Iván finding out that Carmen and Chloe are his mother and half-sister respectively, the latter drunkenly admitting to it. After hearing this shocking piece of information, he avoids both of them.
Iván gives Chloe the silent treatment, not even bothering to at least say "good morning" when she moves in with Raúl, which surprises Chloe. She tries to defend herself by pointing out to Iván that she's as upset as he is about her mother's lies.
As for Carmen, she texts Iván asking to meet up. Once there, you can see how Iván is feeling right now when having her face-to-face for the first time since he learned she's his mother. He doesn't yell at her or anything, instead calling her out in a Tranquil Fury mood for not having been there for him all these years and expressing his disappointment at Carmen for only going after his father's money, abundantly making it clear he wants nothing to do with her. Their conversation finishes with Iván asking how much money does she want. At first, Carmen hesitates, but eventually gives in and asks Iván for two millions and he leaves.
Rocío and her mother arguing. It doesn't help that everytime she wants to know about her deal with Isadora's father, she shamelessly lies to her. Then, she asks her friend Isadora, who wants to know nothing related to her parents' crimes and simply tells her to distract herself and let it go. That is when Rocío brings up pact they made about justice not being fair to them, arguing that the people responsible should pay regardless of who they are, even if it's their own family.
Nico kicking Eric out of his house after finding out he burnt his meds and relapsed again.
Later on, Eric walks on the streets aimlessly and hears voices in his head insulting him and saying the world would be better off without him in it. He looks so visibly broken that he thinks he doesn't deserve to live and goes up a roof to attempt suicide.
Nico talking Eric out of killing himself, explaining that he wasn't angry at him, but Nico was actually angry at himself because he didn't know how to help him.
After the arrest of Isadora's father, Dídac sees her coming back and stands up hoping they resume their relationship, but Isadora shakes her head at him signifying that she still refuses to change her mind about breaking up with him.
Honestly, the entire traumatic events, fights and incidents at Las Encinas post-Season 3. If characters who left like Guzman, Lu, Carla and Polo (who were antagonistic themselves) saw what their school has become throughout the rest of the seasons and the previously mentioned things like that are still happening, you can just imagine how heartbreaking their reactions would be, especially taking into account their genuine growth as better people.
