Tee shirts were not allowed per dress code when I was in school. The one rebel without a cause that was always flaunting the rules was eventually pulled out of school and sent to a military academy by his rich mother. He was trim, proper, and highly subdued next time we saw him.
When my daughter was in HS 3 years ago, the old tshirts with messages that were allowed there the sports teams of the their school or the names of colleges.
Anything else what forbidden and notes were sent home. That even even included the logo for Major League Sports Teams (too disruptive).