I know when I was in HS I wore, nearly every day, stirrup pants or 'leggings' - but those were ones that were the slim cut cousin of a yoga pant. Leggings today are often more akin to the footless tights my dance friends were always wearing. I think the dividing line, fashion wise, is if you have to gather and stretch them to put them on (like pantyhose) they are an undergarment. If you could wear pantyhose WITH the pants, and have room between them and the pants, then they are an outer garment.
For schools, I'd say they had to have a minimum of 1/2 inch of extra fabric on the sides that didn't require stretching to measure. That would also cut out the skin tight jeans, but I'm seeing no real loss - the friends I had in HS who wore them spent more time thinking about how uncomfortable they were than about class.
Undergarment style ones, under a skirt or long tunic? Huzzah, no more flashing underpants in the wind!
Undergarment style ones, with a short top? Um, can you put on some clothes, please? I think you forgot your pants this morning.