Dress codes in schools differ. My school requires collared shirts, while others won't.
Our school calls the collared shirt requirement the "dress code"
If you look in to any student handbook you'll find an outline of what you obviously can't wear. I guess you could call that the dress code but in general dress codes are very specific.
I also assumed that since I'm not a superintendent for a school who is actually writing a dress code that people would have the common sense to discern what they consider reasonable attire and unreasonable attire and make their conclusions from that but I could maybe list them out.