I don't have any nerdy shirts, I'm more of a plain shirt kind of person.
My roommate on the other had has a couple. She is a math education major, which at our school is in the school of science and is technically a math major. She has one shirt that says "Math is √ ", that looks more like a check mark but it's supposed to be a square root symbol, so it really says "math is radical". She has another one that is a large smiley face made out of a bunch of math symbols.