Griding death.
I forged some paperwork and got accepted as a substitute for an adult ed class. When I was finally called in, I showed up with four objects: a strong door lock, a three-pronged garden cultivator claw, and two earplugs. Once I took attendance, I locked the door, put in my earplugs, and began scraping the chalkboard. I watched everyone collapse on the floor and beat their heads against solid objects. Whenever a hero tried to run up to me to stop me, I'd whack him or her in the face with the garden claw. After about fifteen minutes, they were all either dead or twitching or completely unconscious. I stopped scraping and walked around whacking them each to ensure complete death. I then broke the window, jumped out, and ran off into the horizon.