What happened when teacher was drawing on the black board?

I think this is in chapter 21. Mrs. Dobbins is drawing a map of the United States on the blackboard, one of the boys lowers a cat from the ceiling; said cat grabs onto the schoolmaster's wig, revealing his bald head…which has, somehow, been gilded by the sign-painter's boy. The boys get their revenge, and school's out for summer.