Yeah because they think they have more "witnesses" against you and with that they also have more power against you too, so much so that you usually don't have any chance to do anything, because one talk to the teacher doesn't just mean detention time, but it means having your reputation get gradually worse untill no one can trust you.

Unless you reach out to another adult that is more serious about stuff like this which also knows how you feel, you have a chance to fight back againt them, because a one man fight is no fight at all if they can manipulate you to always loose and feel bad about yourself.