Because they're sexy and there's no panty line if your wearing leggings or tight jeans.
Some will, others won't.
It's ALWAYS ok to wear your mom's thong to school, but only if it's sexy!
The only time it would be wrong is if anyone knew what type of underwear you have on. Underwear should be your 'secret' and because of that, there is no wrong time.
It feels pleasurable, sliding up your butt crack