The lawyer problem. If the school has no restrictions on dress, they will, without question, have girls showing up dressed like Kim Kardashian and boys wearing t-shirts that say "Suck My D***, B****!"
When schools try to make rules to prevent this, they are threatened with lawsuits for viewpoint discrimination. In order to maintain decency without saying so, exactly, they have to come up with some kind of arbitrary point, such as "logo size."
And lawyers are concerned about logo size exactly how???
Dress codes to maintain decorum that keep the school from looking like a dive bar get a thumbs-up from me. Logo size...not so much.