Dude, you sound like you haven't even so much as seen a naked breast in your life. What's with the inquiry about STDs? If you're a big enough nerdy assbite to prefer seclusion in a room with a bag of Cheetos, Counterstrike, and your right hand to keep you company as opposed to a party with friends, then you have other things to worry about.
Here are some statments to debunk whatever skewed view you have of highschool parties:
1. They are not massive orgies of Caligulan proportions. Depending on the size of the party, a few people may make out. If it gets really wild, some guy and his girlfriend might get busy in the little brother's bedroom.
2. High school girls, even drunk ones, will likely not strip naked and demand to be rutted on the floor in front of the entire crowd. Stop watching those episodes of College F*** Fest. That sh1t doesn't happen, not even in College parties.
3. You'll only have to worry about STDs if you ride some skanky ho bareback, which, given your apparent level of social ineptitude, will not even have a likely chance of happening until you are 35. Granted, assuming you DO get lucky via a pity screw from the fat chick, you still have a chance of contracting contact STDs such as warts, if she has 'em, regardless of protection.
4. No one will rape you. I promise.
5. No one will force drinks on you, lace your apple martini with roofies, or stick you in a room and demand you hotbox with them.
*edit*
6. If you hear sirens, bolt. Regardless of whether you drank water or everclear, or your proxmity to any illegal substance, you WILL be detained and your parents called.
7. Go with friends. If you know nobody at the party, of course you'll have a horrible time. If you don't have any friends, or don't have any friends who don't have incurable GAD, just go and hang out for MORE THAN 10 MINUTES, ya wuss.