I get your point, but my wife is 5'1" and 98 pounds.
She is a zero P, very fit and I guarantee you she would not wear tights out to school. I'm no prude, but I don't think they're even reasonable for school and no matter how fit you are, you will look fat wearing them.
Maybe she made the point in an inartful world, but there is some over sensitivity here that cannot be denied.
You look fat when you wear tight, stretchy clothes unless you're an unhealthy level of skinny. It's sound advice to tell young women not to dress to look bad and I don't find it body shaming at all. It's common freaking sense given to kids by their teacher.
Isn't that what teachers are for?
Next thing, kids with smokers for parents are going to be offended when the teacher tells them smoking kills.