Question

Many of my friends litter in the class room .how can i stop them

Answer 1

you should complain to the teacher but before that u should try to convince that they should not litter in class

Answer 2

If your friends litter in the classroom, first you tell them not to litter in the class. Even thou they didn't listen your words, then you start to pick up the litter and throw it in the dustbin. do it a couple of times and your friends see and learn from you, and they start to throw the litter in the dustbin.
Hope it helps you.....