Laura Barns' suicide. Especially since nowadays, the number of suicides due to bullying has significantly increased.
There's also the video she posted before her death (which Mitch sends to Blaire) where she basically invited all those who mocked her to watch her kill herself. It doesn't help that the video is similar to a video that the real-life Amanda Todd made before her own suicide (except of course in Todd's video, she asked for people to help her, which proved futile).
Laura's Facebook memorial (part of the viral marketing for the film) reveals that she once saved a little boy from drowning. What pushes it into Tearjerker territory is the offhand comment that she probably never told anyone about it. Either her self-esteem was so low that she didn't think anyone would care, or she was so determined to look like an Alpha Bitch that she didn't want anyone to know that she had a soft side.
Through an Unsent message, Blaire's very fleeting alluding to Laura's uncle, who was implied to have contributed to Laura's issues...
Even worse: Blaire was such a close friend that Laura had confided in her about the abuse. Laura had trusted Blaire over anyone else, and Blaire betrayed her in the worst way possible.
The "Never Have I Ever" game. Adam, Blaire, Mitch and Jess' dirty secrets are all revealed to each other by Laura's spirit, and the four friends proceed to yell and argue with each other, and eventually come to the realization that they've all either stabbed each other in the back or strongly contributed to Laura's suicide. Or both.
Jess's reaction to learning Adam tried to trade her life for his with Laura.
And in a blink-and-you'll-miss-it moment during the commotion caused by Adam revealing that he had slept with Blaire, Jess says this:
Jess: Any other guy I liked treated me like shit!
Mitch finding out that Blaire cheated on him with Adam. More than once.
Jess, Mitch, and Blaire's reactions to Adam's death. Made even more poignant considering the fact that Adam had been a major Jerkass throughout most of the situation. While the scene afterwards is focused on Blaire and Jess as they try to figure out on how to save the latter, if you look at Mitch's cam, you can see him slumped in the back of the room, sobbing profusely. Regardless of him sleeping with his girlfriend, it's clear that Mitch still sees Adam as his best friend.
After Jess dies, the lights go out in Mitch and Blaire's houses, and they proceed to tell each other that they still love one another, just before the "Never Have I Ever" game resumes. It's made worse by what Blaire does afterward...