Idiom: A class clown
Context #1:
Jill: Have you met John, the new student from Texas?
Cathy: Yeah, he is so funny. His first day in class he was doing an impression of President Obama and everyone was laughing so hard.
Jill: I heard about that. I guess he has become the official class clown.
Context #2:
Tom: Man! That accounting class is so boring.
Chris: Yeah, I know. Everyone is so serious.
Tom: We need to get a new student who can be the class clown.
Chris: Definitely!
Meaning: "class clown" is a term used to describe the student in a class who is really funny and makes people laugh. The class clown is usually joking around and doing silly things just because that is his/her personality.
