The bathroom.

You try to think about your options, but soon your mind is clouded by the un-ignorable feeling of something rough rubbing against the oh so sensitive glands of your cock. You look down just in time to see your penis twitch once again into the fig leaf struggling to contain it.
You're undeniably at full mast now, and your 'costume' is just barley containing your tip. Most of your shaft, from all directions, is visible. Not only that but your family jewels have lost all support and slipped out into view. You quickly cup yourself with your hands and thank god that everyone is still distracted by the stumbling drunk still struggling to get back to her feet.

You decide that you can't go to anyone looking like this, especially Chris or any of her friends. They'd never forget the sight of your dick practically exploding out of a tiny fig leaf — let alone ever let you live it down. The only option left is to find somewhere to cool down. Somewhere far away from all the body paint smearing zombie girls, horny drunk teachers, and especially the camera happy party goers. You've already conceded the fact that pictures of your butt are going to be spread through the girl's locker room (most likely by Chris and her cohorts), but you're not letting some lucky girl get a good angle on your shaft or coin purse.

Heads are starting to turn back your way as people start to remember there was a cute near naked boy in addition to the cute near naked teacher. You take this as your que to get out of there as fast as your bare feet can carry you. You make a beeline for the first floor hallway praying that there's a bathroom in one of the many many doors lining it's walls.
Of course you very quickly are hit by a wall of costumed party guests, but you can't slow down and give anyone a chance to get a clear view on you.

"Sorry! Excuse me!" You repeat as you bump and slip past people.

"Holy shit are you naked?!"

"N-No I'm just-"

"Sam is that you? Wow, this is the first time I've really noticed you."

"No way that's Sam, he'd never show up to a party with his butt out. It has to be some male stripper Rebecca hired."

Fuck! You curse to yourself as you lower your head and push past those girls. "Sorry can't talk, have to go somewhere."

"Did someone say male stripper?!"

"No way where?!"

"Wait, Sam is stripping?! The boy from Miss Lee's homeroom?!"

You finally squeeze past the bulk of people as whispers and murmurs of adult entertainment and a naked boy who may or may not be a kid from school named Sam grow louder from behind.

Just as you predicted only a couple doors down the hall was an open bathroom. You quickly dash in before slamming and locking the door behind you.
All ideas of you just playing it cool and sticking around until Chris and Shannon get bored have gone out the window. This was way more humiliating than you could have ever imagined. Having class mates, let alone teachers, recognize you is just too much to handle. It's really starting to hit you just how many of your peers are at this party.

There's no way you're going back out there. You don't care how much Chris will mock you for chickening out, you're going to have to find a way out of this party. Even if that means taking the backstreets home.
In the mean time you lean over the bathroom sink and catch your breath. Your face is flushed with coursing blood. Even your bare chest has a little visible blush from how fast your heart is beating. You lean in to turn on the faucet when you feel the cold marble of the counter press against the tip of your erection.

Your eyes go wide and you look down to see your bare naked penis pressing into the counter.
Your fig leaf is gone. You quickly scan the floors of the bathroom in hopes that it slipped off as you came to a stop.