So the school knows the child has ILLEGALY consumed alcohol and then turns them on the street??? I think this is where the lawyers would have a field day. Assuming the child consumed enough to blow .06, gets denied entry to the prom due to having an adult administer a breathalyzer, gets turned out on the street, gets back in the car, drives away, and kills themselves and someone else. "Why wasn't Johnny at the prom?", "Because Mrs. Smith gave him a breathalyzer and he was nearly legally drunk so he was turned away from adult supervision and told to leave....." I don't buy the legal protection aspect. They know the kid consumed, they have to report.
Back to my personal example - cold medicine makes you blow. Now you have a kid being denied entry to their prom - in front of their peers - because they have they have a cold?