A group of young hoodlums planted seven makeshift Drano “bombs” outside a Washington Heights public school over the weekend, authorities said.

A janitor at PS 178 discovered three soda bottles containing the caustic substance and presumably aluminum foil — which if combined with the liquid can explode — outside the school on Ellwood Street at around 11 a.m. yesterday, officials said.

The FDNY found three more bottles under a nearby car, and cops later found yet another bottle under a second vehicle.

A witness told authorities that he had seen kids leave the bottles the night before.