I suppose it doesn't matter which building you use, but I've found that it works at the Mages Guild in Bruma...
Enter and go left and up the stairs by the guild leaders room.
Jump up on the little wooden rail facing the set of stairs going down to the floor below, if you put sneak on and jump you'll notice that you can jump very fast and while you're doing that, the person in the next room is close enough for you to sneak and gain points for it.
My acrobatics is at 101 and my sneak is at 100... the acrobatic water jumping comes in handy once in a while.