Except as we all know, the school is not legally liable for the kids safety... Think they are? Let Little Johnny break his leg at school or something and then go ahead and expect the school to pay for it.
When my friends and I attempted to play tackle football at recess when I was 12, we were yelled at. Why would the school have rules banning rough play on the playground (which we attempted to ignore until we were caught) if they felt they wouldn't be liable for the safety of students?