This is what I think- The parents are acting like that because no one is showing them the consequences of picking up their kids late, in our school if they take longer than 15 minutes after the bell to pick them up, the kids go to after school care, and the parents are charged for it( we are a private school) , if they dont pay, kids are not allowed back until they pay their dues. Don't let the parents run the show, make some rules if there are not ones already and show those parents what it is going to be like if they are not responsible for their own children!