Around here, they usually just arrest the person who had the party. And there isn't really anyone providing the alcohol other than the stores themselves. There is a little grocery store in town that will sell to a couple minors.
One time a couple of my friends were at a big house party, and were in the front room looking out the window at the cop cars in the driveway. They were whispering to each other very loudly, "Should we run?" "Yeah man, we can go out the back or something..." The answer they got was, "Don't bother running boys." Turns out, the cars had been there for a couple minutes, and the cop was right behind them, lol. The cops got the info of the girl having the party, dumped all the alcohol, and left. Apparently they missed a case though, haha.
Another time, my friend had a little get togethor, and had everyone park in or behind the barn. The neighbors called the police to complain about noise. They didnt have any music or anything playing, but the neighbors needed an excuse since they saw all the cars pulling in possibly. The cops knocked on the door(didnt see the 5-6 cars) and asked my friend to keep it down, and left. No one got in trouble.
Guess they don't care a whole lot, as cops drink too, and drank when they were young most likely. They will usually just ticket the person with the party.