Guns on campus, of any kind, is a no-no, period.
I would be appalled if someone were to let their child play with a toy gun after school, on school grounds. For so many reasons. My husband and I are former veterans and know what guns can do. We do not allow our son to play with pretend guns. He's four, and when it comes time to get a 'squirt gun', it will be an animal shape. If he picks up a stick and makes a 'gun' out of it, this is still different than sanctioning this play in a public area with a prop that actually looks like a gun. (Oh, and you know I would stop him from playing that way in a heartbeat amongst other children. In one's backyard is one's choice; imposing that sort of play on other children is quite another.)
I think the other piece of this is, there's nothing other to play with guns than "killing" games. This imposes a game on other children that they may not want to play, or that their family is morally opposed to. (Yep, that would be me.) These aren't welcome. I'd address this with the teacher, the principal, and hope that some strongly worded newsletters made it home. Teachers should have to be 'policing' the schoolgrounds afterward for it to be a safe place.
These parents should also have enough backbone to tell their kids "these toys are for home". Who's in charge, anyway? It is NOT okay for kids to play guns in public. Just my opinion.
I am going to add, in response to those who ask "who are you to tell me what toys I can give my kids?" this reminder:
Gun play is designed to make one person a victim. You are welcome to give your kids weapons to play with if you do it in your own house, on your own property, where *your* kids who like to play with guns are the only ones affected. Just like my son likes to dig up my yard with shovels, but I don't let him do it everywhere else. Context.
Also a short heartbreaking story: when I was in my 20s, I dated a police officer for a short time. He was on duty when a person was brandishing a weapon and didn't put it down after several warnings were given. He fatally shot this person and yes, they had a toy gun.