I'm coming at it from the viewpoint that if you're posting this here, you ARE thinking of running an RPG. If that's not the case, everyone else's points above are great -- and they're still pretty good points even if you do want to run an RPG.
For kids that age, I'd suggest a couple of things:
1) Keep things as bright and cheerful as possible. Many of the kids might want to work out their aggression in a dark game, and heck, that might be helpful, but it runs the risk of blowing up in your face if someone in authority decides that you are making antisocial kids who are going to cause trouble.
2) Keep things as unrealistic as possible, for the exact same reason. I would, frankly, not run any game that let the PCs use a gun. So d20 Modern is out. (Again, you know better than we do what the situation is like there. I could be wrong. But with all the concern in the U.S. and Canada about antisocial teens and guns, if I were running a game at school, I'd keep far away from anything that might cause someone like a principal to even SUSPECT that this was a chance for some kid to work out his aggressions by shooting at people with imaginary guns.
(Note: I don't think that that's a valid complaint. I'm just saying that unless you really want to fight that battle, this is what I'd do.)
3) Since you'd be running a game for kids of various age ranges, I suspect you'd be better off running games where most of the work is on the GM, not the kids. Your ideal game puts most of its complexity in character creation and lets the game itself flow pretty easily in play. (And thus, you'd make characters for most of the kids, save the ones who wanted to do it themselves.)
4) You might also benefit from a loose, episodic format, so that your game isn't ruined by variable attendance on a weekly basis.
With those considerations in mind, I'd suggest either D&D with a dungeon-hack and puzzle theme, with as few humanoid monsters as you can use (so that they're slaying dragons and beholders and giant spiders and not goblins they can name after the jock who picks on them in gym class), or Mutants & Masterminds with a strong Silver Age feel, making sure that the heroes are really heroes and that the villains are your typical "I'm going to blow up the world" bad guys.
Simple, fun, and when you bring in feats like Power Attack and Expertise and such, you can even turn it into a pretty good mathematics and probability exercise. (My wife is not a big math fan, but she eventually warmed up to it when I showed her how to use Power Attack to maximize her damage output per round, based on percentage chance of hitting at least once and what that damage would be.)
Those would be my suggestions. They're not what I'd suggest for every campaign, but for a game like this, where it is important to avoid any appearance of impropriety, this would be what I'd recommend.
Good luck! I hope it goes well, whatever you end up deciding.