Needs to be age delimited (U) with ID cards. Pick a date that most can agree on and roll with it. Say Sep 30 - give the parents of kids with Oct and Nov birthdays the option of playing "Up" with the friends/schoolmates if in a school that goes till Dec 1. Playing up is not a problem, it playing down that people have an issue with and with grade based systems, there is too many variables to skirt the system.