What to do about someone who litters?

[Mk]

"Hey, I think you almost accidentally left this trash there". Mention that you're just looking out for him so that he doesn't get in trouble by the teacher.

I hate that stuff. It's lying to make people feel better. He did not "accidentally" leave trash there, nor does the dude care about the law. If you're going to use words, just tell him to clean up after himself.

Better yet, clean up after him regardless. There is no reason to say because someone else left trash lying around, that everyone else lacks responsibility to clean it up. When he sees someone else cleaning up his trash that is not employed to do it, that makes a realer impact than words.