I'm with Irena on this one. I've had to intervene several times on my DD8's behalf. I've told them that DD should not be prevented from accessing any book in the school library that she wants to read.
The last time I said it, she was in 1st grade, and when I brought it to their attention, they assessed her reading and comprehension levels, then increased her access to different color-coded levels that included the ones she had been reaching for. They also had a librarian who would allow a child to bring her a book they normally had no access to, read a few paragraphs, ask a few questions, and let the child have it if they did so satisfactorily.
Now they have a new librarian, and DD is complaining again. The only reason I haven't intervened is because DD's gifted language arts teacher has a small library in the classroom, and DD has found books she wants in there.