Bully
(2001)

Bobby is murdered. Life goes on for everyone involved. However, in the end, someone cracks and sends in a tip to the police. Everyone gets caught, arrested and sentenced for their involvement in the murder. The trial isn't shown, but a list of their sentences is shown:

It is never truly revealed who sent in the tip to the police, but since Heather and Derek got the lightest sentence, it can be assumed that they are the ones who snitched.

Thanks Salvatore!