The same thing happened to DS when he was in KA and K. He wasn't allowed to "venture" to the chapter books rows but could only browse in the picture books section. He was equally upset and didn't buy anything. The fair was mended by parents volunteers. As parents are allowed to shop after school, I met DS after school and let him picked whatever he wanted there. I made a comment to the cashier (also a parent volunteer) and also to his home room teacher (I think at the school's corridor another day).
For your case, I would do what Kriston has suggested. Better talk face-to-face with the teacher.
