If it's a big cosplay party, I suggest you buy a wig. 50% of cosplay is wig afterall. There's enough time for a wig to come in anyways.
If you posted this, I'm guessing you don't have time to buy a cosplay??? idk???
Here is a list of simple cosplays: http://www.cosplay.com/showthread.php?t=243782
Generally school uniforms are always available on ebay and quite cheap too. Pretty much 60% of the female characters have similar school uniform.
Homestuck cosplays are relatively simple but don't look halfassed.