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My German teacher was also a chemistry teacher. She had an annoying habit of pacing around the room. One day I mixed up a batch of "contact explosive" and sprinkled the wet bits around the floor. As it dried, it became unstable and "popped" when she stepped on the bits. She was no fool. The second time she heard a pop, she sent the whole class to pick up a copy of a discontinued German textbook that was being given away to get it off the school inventory. Lots of popping followed. Aside from some tiny iodine spots on the floor, no harm done.

My chemistry teacher made a mistake in class (some years after my attendance). The "demo" with sugar and potassium perchlorate detonated and blew off the end of the lab table. Not good. The thermite class project required gathering lots of red rust and aluminum cans. We spent a couple class periods chopping up enough material. A small sandbox was placed on the cement walkway outside of class. The materials were mixed, a magnesium strip added and lighted. We were rewarded with a nice blob of very hot steel.