How about a design contest going forward? Aka, maybe every 3 months or so have people post their designs, users vote and #1 gets used for that 'run' of t-shirts.

You could even tie the contests schedule to the Y class schedule so each class can sport it's own unique design... and also generating more awareness of another application cycle opening soon. The shirts would remain open to the public however (perhaps a unique color given to only Y accepted users).

I'd also add a Hoodie to the choice.