Utility Basement B is a storage room.
Principal Skinner tricked Nelson, Jimbo, Kearney, Dolph and Bart into the room by promising them mountain bikes. Skinner locked them away to avoid problems during the latest inspection by Superintendent Chalmers.
However, after the escape of Bart, Skinner forgets completely about the rest of the boys until the end. By the time Skinner realized this, it can be assumed many days (if not weeks) have passed by. The boys are still alive, though, and it's implied Skinner gave them the mountain bikes he promised.