We recently came back from touring some colleges/universities with our kids.
My son is a junior in high school and our daughter is a sophomore.
I read an article some time ago that reported kids can usually tell within eleven minutes of the start of a college tour if the school "feels right."
The positive T-shirt sign is another way of knowing their interest.
If there's no desire to go to the college bookstore after the tour to purchase a T-shirt, we've found that the school is no longer seriously being considered.
It's a nice non-verbal clue.
No explanation is needed. If the school, for whatever reason, is a bust, why would one possibly want to wear a T-shirt?