Huh? Schools have dress codes that ban jeans??? Do you mean private schools with perhaps uniforms or public schools?
When my daughter was in high school, they disallowed jeans with holes in them. She had ripped a pair of her jeans, it was the "in" thing then, wore them over leggings so no skin showed. At 8:30 am, I received a call from the school that she had been pulled from class, I needed to come get her to change clothes. When I arrived, I met with the principal (a man) and questioned it. He said it was a distraction for the boys.
My response was "On Fridays, the girls on the cheer squad and dance teams wear those short little skirts with ruffled panties, low cut tops, and parade around all day exposing LOTS of skin. That isn't a distraction? Why is that allowed?" I also told him they were using sex (the cheer/dance teams) to sell violence (football). He said there was a committee of parents who made the dress code and perhaps I should get on that committee. I told him I would do that. Have someone call me. Never did get a call.
Edited to Add: The principal's three daughters were on the dance team...