It's driving me crazy

I'm looking for an old children's book my school library used to have. It was pretty surreal, about a boy who falls through a puddle and lands as an indentured servant to a wizard?

Any help would be appreciated. 

Yes it was, this kid and some girl go to a magical land where there are a bunch of puddles and for each one you go into you go into another world or some shit. I remember reading it a while back.