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?
Among other reasons, because they dont WANT you to get hurt. Has nothing to do with liability - it has to do with natural adult responsibility for children.
Believe it or not, there was a time in this country when people took responsibility for their own screw-ups and didnt blame someone else if they fell down and skinned their knee. Oddly enough also, even during that time, people still admonished others not to do something stupid, which could cause them to fall down and skin their knees.