Don’t show up naked.
We had to be clothed. Which is fair enough, as it is quite cold in the UK.
Primary school (ages 4 to 11) - We didn’t have a uniform.
High school (11 to 16) - School uniform. Jumper, tie, white shirt, grey trousers, black shoes (not trainers). No jewlery. For girls, skirts couldn’t be above the knee (this was rarely enforced though unless someone was taking the piss).
Not super formal, but I remember things like the “two finger length rule”, all straps must be at least two fingerlengths wide.
Grew up in the middle of the Bible belt.
Every once in a while a teacher would tell someone to take off their hat inside the classroom. Kind of like a gentle reminder that the teacher was still the authority in there. Does that count as a dress code?