Phase 1: Turn on all lights in back/front yard/porches.
Phase 2: Turn on sprinkers precariously close your their house.
Phase 3: Plant poison ivy in all prime urinating real estate.
Seriously though, call and tell them you smell marijuana and that you see people you are pretty sure are in high school that are drinking. All you have to do is be vague enough to not be in trouble for false accusations, but entice the police to respond. If all else fails purchase some weed and stash it somewhere in their backyard and then call it in.