Would you go as a school girl? I know it's the obvious "simple" cosplay but it's still always a good look that you can pull off even with minimal planning. What time will you be going to Comiket? I could give you some tips on which style of uniform you can manage in your time slot if I know. I think I can honestly say that I have done almost every type of school uniform and while I generally spent what was a lot of time on my costumes (even the really simple ones, but that's just cause I'm a perfectionist
) there are definitely ways to cut corners and still come out with a great costume.
But like Eleryth said, you could just goth up or dress up in J-street fashion. It's not the most important thing and you could atill have a lot of fun!