1) Double Dare contestants, but good luck finding a legit tshirt.
2) Hans & Frans - grey sweats and a weight belt, very simple.
1) Hugh Hefner - dark robe, fake penis maade of slightly stuffed pantyhose w/ fishing line tied to make the tip, other end of fishing line tied to wrist at appropriate length to make you 'aroused' when drinking or shaking hands.
I remember I had a friend in high school who dressed up like the WWE character, The Godfather. Remember him? He was so 1999 because Godfather was most prominent in 1999. He had the hat, the shades, the white pants, and he even wore a green vest without wearing a shirt. That doesn't bode well when you attend a Catholic school though, where all administrative staff are extremely anal about dress code.
Don't go as a group. It's a pretty lame move - plus, it's hard to differentiate yourself. C'mon. It'll be hard for a girl to approach you or describe you to her friends if you do that.
I'm not giving you any suggestions, but I'll tell you what I'm going to do. I'm dressing as John Cusack's character from that movie where he holds the boombox over his head and Phil Collins is on the box. Girls love that movie. Of course, I'll have to actually see the film so I can get some of the mannerisms/character names down.
Halloween, as all of life, is just about impressing women.