I'm entering freshman year in High School and thank God the dress code is better than in my Middle School. They only allow collar shirts, no logos, no stripes, nothing. Only single-colored collar shirts. Apparently it's because they say:
"We don't want students coming to school wearing t-shirts with designs on them because it causes other students from participating in class. They're distracted from learning and focus on the shirt."
Which is the absolute stupidest idea I've ever heard. Assuming that all the students went to school with unique designs, it's pretty dumb to say that people would be staring at each other's shirts. Besides, I wouldn't know why they don't allow single-color t-shirts anyway.
This is also particularly funny because they allow custom and specially designed backpacks which cause a lot of distractions. They even had a yearbook contest about the best backpack.