About the time I got to the point in school of needing to send documents (mid-00s), flash drives had come down in price that everyone had them, and by 11th grade, even we had school emails.
The majority of my use of them was that I cut my computing/networking/programming teeth on all the 90's and early 00's computer hardware that my dad had hanging around. I had piles of floppies and no flash drive so I made do with what I had available.