The last chapter/episode. Sakamoto is leaving Gakuban High school to go travel overseas, everyone he has ever befriended started shouting goodbye to him tearfully from their classroom window, saying how they'll never forget him and Atsushi promises to study harder so he can meet him again someday.
In the previous three chapter arc, Atsushi's shame over being unable to meet Sakamoto as an equal, a goal that's implied to be at least partly motivated by his crush on Sakamoto, being twisted by Fukase into a desire to attack Sakamoto, is pretty genuinely upsetting, especially when Atsushi snaps out of Fukase's brainwashing and breaks down. There isn't much about this subplot that's played for laughs.
The implications of the theory listed above retroactively makes the very premise of the story pretty poignant.
When everyone says goodbye to Sakamoto, he noticeably has a sad expression on his face which is clearly seen since he was the only one to escape getting pied in the face, even holding a pie in the process. However, just as he leaves the school, when asked who he is, he is shown to have the pie over only his eyes. It wouldn't be a stretch to say that perhaps he used that pie to hide away his tears.
Episode 7 had an unexpected Mood Whiplash during the Class Sports Day. Of course, Sakamoto, with his shoelaces tied managed to overcome his problems by running like a cheetah. But while he had his plans, he didn't notice that he was holding his baton on his mouth, so his saliva made the baton slippery that Yagi failed to get ahold of the baton. Just in a short moment, everyone then started badmouthing Yagi that she failed Sakamoto, while she continued to run while crying, and this resulted Class 1-2 not taking the 1st place despite having Sakamoto with them. It took Sera, with a bit of goading by Sakamoto, to make an even more ridiculous moment that everyone forgot Yagi's failure, but during that time, she just looked so pitiful.
Hayabusas father, Sakamoto, and Hayabusa find out his girlfriend and potential wife is nothing more than a extortionist that attempted to use other thugs and may have been fooling Hayabusas dad for months. Considering that Hayabusas mother is never mentioned, and acts as The Caretaker to his little brothers, the implications only make it worse once that chance for their family to be more stable vanished. Thankfully, Hayabusa takes the event in stride, learning to make up for what the rest of his family cant do and vice-versa from Sakamoto.