They weren’t - when I was young, boys used something like this and girls used something more like a tote bag. Backpacks were strictly for hiking.
That was one of the shibboleths for rail fans. There was a locomotive works there and ‘bean’ or ‘Peru’ told you how much studying they had done.
Some kids I went to junior high with carried books in an open top canvas bag with shoulder straps. Technically a backpack, but not in the way we think of it today. More like a shopping bag with shoulder straps.
I have never been able to study from a Ebook. Casually reading fiction 
on a Kindle is relaxing. Studying from a Ebook breaks my concentration.
I just bought a book on ***Python for a online class that starts July 8. I think it’s available in Ebook. But I need a real book to read and study. My yellow highlighter near by and ready to use.
Gosh, I’d be in serious trouble in todays high schools. I tried to finish my homework in study hall. Leave my textbooks in my locker. I took them home to study before tests.
*** Bill Lubanovic Introducing Python
Excuse me, I meant May 20th.