Our school did this a couple of months ago and there were some awesome costumes! I went as Pinkalicious and a colleague went as Purplicious...pretty easy to do if you've got a lot of those colors in your wardrobe. Some other teachers went as the three blind mice--they just wore grey/black sweats and put on sunglasses, ears, and tales. We also had a lot of Dr. Suess characters. Our principal wore overalls and went as Farmer Brown from the Click Clack Moo Series...the related arts team dressed as Harry Potter characters...One of my favorite costumes was the old lady from There Was an Old Lady Who Swallowed a Fly. The teacher put on a wig, a huge dress, stuffed her belly, and pinned felt characters from the story to the front of her dress
If I think of any of the other good ones, I'll post them too! Have fun!