My Gym Teacher Is an Alien Overlord by David Solomons

Zack and Lara have superpowers. Luke has new school shoes and a burning sense of resentment. He KNOWS that aliens disguised as gym teachers are about to attack Earth but will anyone listen No. So one dodgy pact with a selfstyled supervillain later and Luke is ready to save the world. He just need to find his trainers.