I observed a teacher who had a pretty good policy during her small group reading/individual working time... each kid has a magnetic name card, and there's a spot for one boy and one girl to be out of the room at the bathroom at a time... if they need to go, and there's an empty spot for their gender on the board, if she's working with a small group, they simply hold up their name card, and she nods yes or no... they put their card on the board, and remove it when they come back.