I think once the school gets to where the dress code is strict enough that you can't find clothes in a local store, they might as well just go to uniforms. At least then there's no question of what's right and wrong.
There was a constant struggle with my daughter's middle school last year with the dress code for girls. "Some" girls would get in trouble and "some" were fine in the same types of outfits. It was so ridiculous.
It was getting to the point where my daughter was coming home in tears because the assistant principal was cornering her to measure this or that and making her bend over (to see how much cleavage was showing). When we went shopping, I followed the school handbook to the letter. If they said a skirt had to come below fingertips, then I made sure it was considerably below fingertips PLUS I'd make her wear leggings under them. She got in trouble three separate times for different skirts and leggings -- yet, girls were always wearing leggings with tunics that were shorter than those skirts.
Was that principal picking on my daughter for personal reasons? Hmmmm, let me think.

Uh, yeah!
The high school has been soooooo much better for this kid. Same district, so same dress code. The high school is just more reasonable and consistent in following it.
Anyway, sorry for the minor rant. I just think you're in for a long haul if that's how they'll do it in kindergarten!