This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / 21 Jump Street
Blindsided: A teenage girl who's being sexually abused by her father, repeatedly tries to get help from police and social workers for her home situation, but they repeatedly fail to help her - because her own, abusing, father is a police Captain and uses his police influence to habitually dismiss her case. It's made even worse by the fact that the girl also has lost her mother already, that the father has recently started abusing the girl's younger sister too, and that at the end he's arrested for it (which might be a good thing in itself, but means the girls now don't have any parents left).
Blinded by the Thousand Points of Light, which was about homeless teenagers. Seeing how hard life is for them and how they struggle to survive is very sad indeed.
A Big Disease With a Little Name, which featured a teenager with Aids (back then a death sentence, and an even bigger stigma) who was shunned by his peers and coming to terms with his dying. The issue may seem obvious and dated this many years later but if you can get through "Just hold me, man..."
Loc'd Out - Part 2 and Draw the Line - Hanson is wrongly convicted for a murder he didn't commit, and then is shown in prison, where of course he has an extra hard time because he's a cop...
The death of Tom Hanson and Doug Penhall.
Jenko's feelings are really hurt when he overhears Schmidt make fun of him.
"You still don't know your Miranda rights, you idiot!"