1) What if the family can't afford it? Do you just deny them graduation?
2) I'm not necessarily saying that they get stolen when they go home. A room full of iPads just sitting there over the summer is a very tempting target for thieves.
To your first point, yes, that's how it worked. You don't pay, you don't get a diploma. I personally had to pay something like $120 to replace a physics textbook that I apparently didn't return at some point, despite the numerous angles 18-year-old me tried to play to get out of it.
To the second, most school districts, as far as I know, have central warehouses for textbooks, so why wouldn't at least a portion of a warehouse be converted to secure iPad storage?
2) I'm not necessarily saying that they get stolen when they go home. A room full of iPads just sitting there over the summer is a very tempting target for thieves.