I teach sixth grade and here's how our team addresses this problem. Students receive a laminated "ticket" with their name on it at the beginning of the day. If they need to use the restroom (they hold up three fingers to signal,) they sign out and turn in their ticket. Students may not use the restroom if it's within 20 minutes of the bell, unless it's an emergency. At the end of the day, the teacher collects the unused tickets and draws a name for a reward.