Our school gets out at 4 and games start at 445. There is no down time between the end of school and game time for us. If games were later, I would institute a study hall where all the football players would come and do their homework before the game (if it were dismissal at 230-3 and game at 430 or something like that). I would never let the kids leave and then come back, then you get into the "I don't have a ride back" scenario and you also give kids the opportunity to get into trouble as well.
Our school is out at 2:50, we kickoff at 6:30. We walk the field at 5, then come in and dress and get ready for pregame. We do study hall right after school, but it can only last so long. Still end up with an hour or more of herding cats.
You can always utilize some of that time to feed the kids. Right after school, give them 30 minutes or so to eat and relax and get their chit chat out of the way, then after that time, it's study hall for an hour, then pre-game chalk talk, then game time. Definitely give them time to eat and be kids, but put a time limit on that and let them know if they act like fools, it'll be shortened.