I don’t think there’s one set of countywide rules. I have kids at two different MCPS schools. One had prom from 8-11:00, the other was from 7-11:00. One school allowed parents to give permission for their kids to be dismissed from school at 11:00 am on the day of the prom, as long as they’d attended the first four periods. I don’t think the other one did. Both proms had a time by which you had to arrive by in order to be admitted, but you could leave either one whenever you wanted. They both had a rule that you could not be re-admitted if you left.