If I was too go to my high school wearing medieval knight armor with no weapons or shield what would happen?

What would happen?

Well, you'd have a terribly tough time walking, that's what.

What is the dress code?

If not related to a school project, the administration would undoubtedly view this as a distraction, and take steps to eliminate it. Speaking of elimination, the business of bathroom breaks might pose a difficulty for the wearer.