When I was a student teacher, I taught a mixed 1st and 2nd grade class (this was when I discovered that I never wanted to teach little kids). At the time, I always wondered why elementary school teachers wore stretch pants under their sun dresses. I mean, the dresses weren't transparent and the stretch pants turned what would have been simply elegant summer attire into something mommy-like. Mind you, they might be mommies, but stretch pants under sun dresses always looks silly.
I discovered why when I taught the first and second graders. On that day, I wore a beautiful blue sundress with a black dappled pattern that reminded me of what the world underwater looks like. And of course, I wore it without black stretch pants.
Have you ever seen elementary school students playing with a parachute? It's fairly common: everyone takes a piece of the edge and you all raise your hands high, letting air fill the parachute. Then, when the chute is at its highest, you rotate so that you're holding the edge still but your body is on the inside of the chute. Pull the edge quickly down to the ground to trap the air inside, and it's like everyone is inside a giant silk house.
The students took one look at the folds of my sundress and thought, "PARACHUTE"!
Three students ran to me, grabbed the edge of my dress and flung it high into the air. Then, they stepped underneath and tried to pull the dress to the ground, nearly pulling me off my feet. Far more worrisome to me was the fact that everyone had seen my underwear and I now had three small children on the inside of my clothes. People have been arrested for less shit than that.
So that's why elementary school teachers wear stretch pants under their dresses. You can't stop them from playing parachute with a dress. The best you can do is avoid child molestation charges.