What/how to act/do at a high school party?

OK, assumption time. You haven't been to any of the parties before: you're not one of the cool kids. You're probably still very beta. That's okay, lots of time to fix that.

Don't sit in the corner. Don't just look for people you know and then follow them around like a puppydog. Talk to people you don't often talk to. Wander around and meet people that don't go to your school - they have less preconception about you. Drink moderately; not too quickly or you'll do something stupid or awkward before everyone else is drunk enough to enjoy it.

You will most likely not be getting laid at this party, especially since in this day and age it's a pretty bad idea to fuck anyone that's drunk, even if they seem to be down. That's fine. Try to approach a bunch of different women over the course of the night. Your goal is to make them feel comfortable but interested while talking to you. Don't overthink it, just smile, and ask questions. What kind of music do they like? Have they seen any shows lately? What did they do over the summer? Which teacher do they hate most?

There is lots of advice out there for kino techniques. Be a little touchy, slow to start but escalate if she warms up to it.

If other people are dancing, dance! "I'm bad at it". Everyone's fucking bad at it, that's why there's alcohol and a bit of darkness.