In case you are one of these confused, grown women, here are two reasons to look into pants. First, leggings are not opaque. They may appear opaque from the front, but when you put a strain on them by bending over (or in my case just stretching them across my butt), the skin and underwear underneath show through. Second, leggings are a tight knit. Knits conform to your shape, which is why people find them so comfortable. However, this means that leggings are showing off every conture of your lady parts and no one, least of all your students, wants to ponder your vagina. If you wouldn't wear lycra bike shorts to work, don't wear leggings as pants.
"But I love leggings!" you say. "How can I wear them if they're not pants?" Well, that's easy. Think of leggings as super tights. You have a cute dress you want to wear, but it's 20 degrees outside? Leggings and knee boots. Sometimes I even wear leggings over sweater tights for ultra cozy legs. For blistering cold, I wear a triple layer of tights, leggings and jeans. Take that, Winter!
While leggings aren't thick enough to preserve the details of your behind when left alone, pairing them with a short dress, tunic, or skirt works. The leggings are thick enough that if your skirt accidentally creeps up when you bend over to remove your boots or try to climb on a high stool, no one will have the time to notice the aforementioned things leggings reveal. The flash will be quick enough any observes will only register the color of your leggings, unlike tights which would fill everyone in that you like polka dot undies. This is obviously not an option for professional attire, but for a casual occasion, break out the leggings if you feel your dress may be creeping into shirt territory. I have a couple dresses that have shrunken slightly, so I wear them in the winter with leggings.