I can only speak for my own school and share our experiences. I teach Y4 and all my readers, including my level 4s are still on the scheme, but we have a vast range of books right up to band 16. These books are much harder than a lot of the books we have in our Library and ensure that the children can access challenging poetry, modern and classic fiction, and non-fiction texts. We know that children will read their favourite authors anyway, so the books we provide encourage more variety and experimentation. We used to have free readers by the end of Y2 and in Y3, but some of the books they were reading discussed issues that were far too mature for them. There was no way we could possibly monitor the content of all the books that were available to them. Keeping to the scheme until at least Y5 helps to ensure I don't get irate parents coming to me about the content of the latest Jacqueline Wilson novel I sent home with an 8 year old! But, children are not expected to read all of the books in each level! We have over 80 titles for each band, so they would never progress!