WTH, 180 bucks for a stupid party? I generally hate the ide of American proms. You're supposed to bring the hottest date, the most exspensive tux and at least a lamborghini to be considered "cool", at least according to TV

P). TV also taught me that there's usualy parents and no liquor. That's no party I'dd like to attend...
Our prom was free, it was held in a castle, only drinks costed money, though not as much. All you needed was a suit/dress and a cool way to get to the castle. I borowed my uncle's car, a BMW convertable, and I still had the suit.
As to your bigger problem: How to spend 900 dollars in a way that gives you the most amount of fun.
Ideas are:
-Your original idea sounded sweet: liquor, the magic herbs and a freaking waterslide!
-Take the booze/brownies idea from idea 1, combine it with an afterparty. There's got to be an afterparty you can sneak to? Maybe someone you know? Just an idea.
-Go to a stripclub. Imagine a pocket, filled with 900 singles...
-Depending on whether everyone likes videogames: An idea brought to me by Yahtzee, and I still plan on doing it some time soon: Take a game, preferably Devil May Cry, take turns playing, kick back and take a drink every time someone defies the laws of physics. When the first bottle's down, ANYTHING looks like physics are defied. Also, awkward deaths become inevitable after 1 liter of finest Scrumpy Jack's. There's nothign I like more about watching someone play than laughing at awkward deaths...