Kegs are almost always filled with beer, so the argument 'oh, it can be filled with soda!' is pretty nonsensical. I have never seen a keg filled with soda before - has anyone else? you can also put water in a car gastank, so who cares?
but the cop should have stopped the breathalysers after realizing whats going on. What a doofus. and a waste of money.
there are all these stupid laws that cops can pull out of nowhere to get you in trouble. I dont think the school should suspend anyone for things outside of school though. Schools have their own law system, they can do whatever they want, you know?