When I was in school, there were no restrictions on what books we could read from the library, but below a certain grade level (sixth grade and below, I think), there was no school library as such. Each individual classroom had its own “library” (i.e., several sets of bookshelves in the back of the classroom with a few humndred books) which were the only books that that particular class could check out. Not only couldn’t students in other grades check out the books from your grade, but the students in the other class for your grade (there were two classes for each grade) couldn’t check out the books from your class. This meant that de facto you were restricted to a certain level of reading material. There was also a junior high school library (for all seventh and eighth graders, I think) and a high school library.
Furthermore, it was somewhat difficult for the students at my school to go to the public library. I grew up on a farm, as did most of the kids in the school district. The little towns in my county didn’t have a library. You had to drive 15 miles into the county seat to get to the county library. Getting into town to go to the library was a big deal. You were lucky if you could do it once a week.