Get a fire extinguisher. Go to the girls dorm and do something that will turn the teacher's attention on you. When they start running towards you, spray the teacher with your fire extinguisher. When they wave their hands over their faces, attack them normally by punching and kicking them. Repeat this process until you manage to weaken them and take their health to zero. While doing this, do not drop the fire extinguisher or there will be a good chance of getting busted.