Just remember that in HS football you have on a Helmet, shoulder pads, griddle, knee length pants and soft pads. Very hard for heat to escape. That is why there are rules in place. Also I can't recall anyone dropping dead at soccer practice due to heat. Happens every year in Football, and those kids are mostly bigger kids with who are out of shape. Lots of water and breaks are the key and most teams have plenty of subs.