class clown
A mischievous or impudent student who frequently disrupts the class with jokes, pranks, or wry comments as a means of drawing attention to him- or herself. Every teacher has to deal with class clowns eventually.
clown around
slang To joke, play, or otherwise behave in a silly way. I can see you boys clowning around back there! Sit down and do the math problems I assigned. The kids are just clowning around with each other in the back yard, if you want to call them for dinner.
like a clown's pocket
Very big, like the pocket on a clown's baggy pants. This one's got a mouth like a clown's pocket on him—don't know when to shut up.