This inspires me to ask about another book I read in the 1970s (probably in 1976). It was a paperback (Scholastic, probably) about a group of 12 or 13-year olds who find out about a sleeper agent for the bad guys (a group called “Cobra” or something like that - note that this was before GI Joe invented Cobra), and that this agent planned to put psychoactive drugs in the water supply in their town, to terrorize the populace. At least one of the kids works part-time (or helps out unofficially) at an auto mechanic shop - near the climax they realize that the villains car isn’t firing on all cylinders because the drug was being stored in the coil for one of cylinders. Any ideas?
looking for the title of this book too! I think I read it in 6th grade so that would be around 1974. I googled " children’s fiction sleeper agent water supply poison"… and got your posting.