Lock a buildings doors and set it on fire. Drive a car into crowded street event. Yank steering wheel of a bus and run it off a cliff. Poison the water or food. Create any one of a hundred poison gasses. Blow up a gas tanker, Spray white gas with an improvised fuel air explosive. The ways of mass killing are infinite and not affected by gun bans as history has shown. There has been several examples of people shooting with bolt action rifles and killing dozens. All it requires is a crowd and high ground.
In the case of a school, the only one who will rush you would be the teacher, which you no doubt would have shot first. In a land with no guns, the man with a large knife is king.