At the risk of showing my age, I did that in business school. There was a period when you could buy floppies in bulk and pricing was maybe 10-20% what they were by ones or twos in the campus store.

Someone else started this first but the floppies they were selling were crap so...

This was also a period when I earned money working in the school's computer lab to help students and there was more than one occasion where a student had their semester of work on a floppy and they bent it or otherwise made it difficult to read though I could usually get most of their info off.

Same here. I get the feeling this was a pretty common hobby. I would also pre-format them and charge a little extra. If you could afford to buy a batch of a thousand or so unformatted single sided floppies, doubling their capacity and formatting them, you could make some pocket change.

how many kids had computers? when I was in HS I was using 3.5" floppy and there were only a handful of us that had computers...

They didn't necessarily have computers, but they used computers.

Only a handful had computers, but there were enough who used the school computers to make it worthwhile. I didn't own my own computer until I was out of college.