regardless of the situation, martial arts or whatever. If one has a legal right to sue, a waiver means dick in most situations.
Waivers are generally used as a way for businesses to convince others they signed away their legal rights.
On the back of baseball tickets is a little implied waiver that says something like there is a risk involved in watching the game like getting hit by foul balls and buy purchasing the ticket you agree to those terms. that said if someones head gets their head cracked in they can and do sue.
its like the sign at the dry cleaner that says they are not responsible for lost items, they are but most people believe the sign.
Waivers are still a good idea to have in place, and to sue a school is a douche move.