Go ahead and hit them back. Think of it like Stick Arena; it's unfair to free kill players without weapons, but if they start punching you or start using it as an excuse to get the best weapon instead of using it as leeway to get the nearest possible weapon, blast them to bits!
Teachers won't fix anything. Neither will saying anything to those girls. Start of by blocking their attacks, and if they keep persisting, hit them back. And since you should hit them back as little as possible, make each hit count. Bad.