If it bothers you, report it (to the principal or a school counselor or to a teacher you trust).

My mom is a high shool teacher and she reports sexual harassment when one student picks on another one that way. If it's unwelcome or if it makes you uncomfortable, it IS sexual harassment. (Even a dirty joke is sexual harassment if it offends you or makes you uncomfortable.)

Last year, she reported a boy who pulled down another one's pants (a "shank").

Yes, the boy was "just joking around" but the victim doesn't feel so good about it, and they need to realize this sort of thing is NOT to be tolerated. (That boy got suspended. Now he and those around him realize what sexual harassment is, and that it is BAD.)