One approach to dressing as a villain for your school's spirit week is to dress in one of the sports uniforms of one of your school's rivals. While it can be the uniform worn by any of their teams, it is best to go with something that will look the most like a costume, such as a football or basketball outfit.
Similar to the idea of wearing another school's football or basketball outfit is to create a copy of the costume worn by their team mascot. For example, if the opposition's mascot wears a wolf outfit, you should try to replicate it. However, since you will probably want to have people cast derision on the "villain," you might want to make additions to make the character less impressive. For example, if your team soundly beat theirs in the last game, you might add a cast and crutches to the outfit.
Try dressing up as the chancellor, who is the head of the school and frequently has to put up with a lot of good-natured ribbing. It is even better if a large number of students do this. This one requires at least two of three things. One is that the chancellor of your school has to have an easily recognizable appearance. He also has to have good sense of humor, or be out of town. If you prefer not to antagonize the current chancellor, you could dress as a previous one, especially if he left under a cloud. Alternatively, you could dress as a opposing school's chancellor.
You don't have to confine yourself to only school-based villains. Since one of the most popular types of literature on college campuses is comic books, you could also choose to dress as you favorite comic book villain. Some of these will be easier that others, since to be Magneto or the Punisher all you need is a costume. For others, like the Green Goblin or Solomon Grundy, you might need the help of makeup artist friends in the theater department.