Well, technically it IS legal for someone under the legal drinking age (18 or 19 in Canada) to drink so long as they do so under the supervision of their parents/legal guardian in their residence. But I'm pretty sure that they have to be at least 16 or 17 in order to do so. There's a lot of restrictions on it, but TECHNICALLY if you are drinking at home and have your parents permission, then you're okay. Of course, given what grade she's in, I'd say it would still be illegal anyway.
Schools have rules, deal with it, and breaking those rules results in -GASP- consequences. If you're going to show up to a school dance or function ? on school grounds, meaning that the school is responsible for the child/ren at the time ? drunk to the point of throwing up, you're going to get in trouble and/or face consequences that you will have to deal with. Even if it was only just a little bit of alcohol, she still drank it. The school's probably just covering its ass more or less.
And besides, from what I remember, school dances at that level were about as entertaining and fun as watching paint dry. I can't imagine being drunk off your ass would make them that much more thrilling. But then again, I hated school dances and I was forced to attend them because I was on yearbook so I'm not the best person to judge.