G.B.
Okay, I get that you miss his old school okay? So here's my answer.
Your child is plenty old enough to follow directions. He's old enough to see his classmates grabbing their reading bags and taking them home. He's old enough to ask why they have one and he doesn't if he doesn't have one sitting there waiting for him. If he didn't have the same thing as everyone else don't you think he'd notice?
So why wasn't he picking up that book bag and bringing it home like every other kid in the class?
The librarian is responsible for the books the kids check out. They scan them and know what age they're supposed to have access to and not. But technically stuff that's "tween" is for age 7 and up, the same as Y7 TV shows.
I'd definitely talk with the principle but I'd also take responsibility for my child. He's not 3 or 4 he's 7 years old and knows the kids were bringing home stuff to read. He should be focusing on his AR books and getting his reading skills built up. Those books should be in his classroom and available for him to check out to read then test on the next morning.
IF IF IF the books in the book bag are his AR books and he hasn't tested on any of them all year then I'd be yelling at that teacher and throwing a hissy fit like she's never seen before.
I do imagine there may be some AR books in that bag but he has to be reading and testing or his grades would be really really low in reading and you'd have noticed before. So I assume he's reading and testing during the day while he's in school.
I am glad he enjoys his after school care, I hope they have a great time and get to play a lot. It's fun to stay after school to play.