My ds's school provided books in a "browsing box" for kids to read. The books offered corresponded to the topics they were studying in science and social studies at the time. When kids had extra time, they could read about Egypt when they were studying Egypt. Books at different levels allow the kids to read at their own level. Even if the higher-level readers started with the easy books, they would quickly move on to the harder ones.
Also, the school allows the kids to test out of material they already Words Their Way. For math, they provide extra materials for the ones who tested out to work on independently. They used Contental Math League questions for this. In order to do the extra math, the students and parents had to sign a contract (I will not brag about doing extra math, I will not disrupt the class to ask about what I am working on, etc).