Hmm.. now that I think about it, they've used the University theme before, only back then, it was called Super Mario Bros University. The shirt I had was a kids' white T-shirt, and it had a picture of the cast of SMB on it, all in graduation hats and holding diplomas, in front of a white building.
I have an old photo of myself in the shirt. I'll do some digging and see if I can dig it up and scan it..