Most girls seem to wear black shorts or spandex underneath their skirts and dresses

Some girls. Maybe a lot where you are.
A few guesses:
- Fashion du jour, to look athletic
- Good for under short skirts so panties aren't seen
- The Spandex acts like a girdle

Lets see, could it be because the girls want to keep "Roaming Hands" away from where the guys want to go?

Most girls seem to wear black shorts or spandex underneath their skirts and dresses

My first guess is modesty. So if your skirt flies up you don't see their underwear or thighs or anything they don't want you to see. As well my wife wears it to prevent "chub rub" or skin irritation caused by their thighs rubbing together. For fashion I Think it works better then a slip because it allows the skirt/dress to fall naturally or as the designer had intended. It could also prevent Visible Panty Lines form appearing through their skirt, or depending on the garment, the Panties being visible.

It also prevents perverts from seeing parts of their body that they're not comfortable with.

There are countless reasons. You can always ask a girl as well.

Most girls seem to wear black shorts or spandex underneath their skirts and dresses

Have you actually seen "most" girls? How do you define girls, any female or females of a certain age grouping? I strongly object to people who make such broad statements where they could no possibly support the contention. I was walking through a mall last weekend and did not notice a preponderance of females attired that way. So your premise that "most" girls dress that way starts erroneously.

Spandex is worn to make a less then perfect silhouette slimmer and trimmer. It is the modern day 'girdle' only in a more simplified form.

I wear a slip and shorts under dresses because I teach Kindergarten and often have students with wandering hands. Until they learn proper behavior, I'm double covered.

I wear a slip and shorts under dresses because I teach Kindergarten and often have students with wandering hands. Until they learn proper behavior, I'm double covered.
And we often sit on the floor or on little chairs.

And we often sit on the floor or on little chairs. Yes, we do. I had no idea how touchy-feely this age would be. I do spend a lot of time crawling around doing puzzles or testing at their level. Shorts work well, but the point of a slip is to not see through the skirt/dress. So, I wear both.

Underwear is just that, to wear under and not be seen. Many girls wear such short skirts, that what they wear under is easily seen.

I think he might be referring to the Spandex worn at or just below the knee, under a very long top or a short skirt.
That's a lot more common than hidden ones.
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If I were a teen again, I'd splurge on the space ones!

My 14 year old wears shorts under her skirts because she doesn't want her skirt flying up and her underwear showing, and also, with shorts she can still be active and not worry about showing things she shouldn't. She can ride her bike to school wearing a skirt because of the shorts under that skirt. She can play soccer or basketball with her friends during lunch break without having to change clothes. It allows her to wear a skirt but still do all the things she likes to do instead of primly sitting on a bench watching others have fun.

Probably because guys like you are looking up their skirts.

Probably because guys like you are looking up their skirts.Oh that takes all the fun out of it... ;) But yeah...guys do that, even the ones who won't admit it.