This is a famous scene from Mean Girls. The titular characters have a secret book in which they write nasty things about other girls in their school. It ends up getting distributed and seen by everyone at the school. The entire school is sent into chaos, which results in the principal ordering all the girls to assemble in the gym, where they are made to write out confessions about nasty things they've said. The girl in the red shirt is a sobbing girl turns out to be some random girl who doesn't go to their school but showed up there for some reason.
Here's the scene: https://www.youtube.com/watch?v=sR528E5_8yI