Threaten them with a teacher.

With more confidence than you feel, you say, "Hey, I don't think you guys should be in here smoking and having sex. If you take off now, I won't go get a teacher in here."

One of the boys advances on you threateningly. "Gonna go get a teacher, huh? You hear that, Manny? The new kid's going to get a teacher if we don't knock it off. You're new here, so maybe I'll give you a pass. I'm Leo and this here is Manny and the guy getting his rocks off in the stall is Sean. And nobody's getting a teacher. So why don't you just do what you came here to do and scram, okay?"