Ds2 started at a new school a couple of months back. He was doing well with his reading and I wasn't sure what group of books he was on but they have assessed him at the new school, and they don't seem too sure what stage to put him on, so anyway.
He's been given the opportunity to choose his own books and has come home with some stage 15s which he finds perfectly readable complete with emphasis, understanding of the plot, etc etc. He's well away.
I got a note in his record from one TA or reading assistant (not sure which) who reads a lot with the class I think, and she put that he ought to be on stage 12, but this is a 15 (!). I wasn't sure how to respond so apologised and said he chooses his own books so I'm not sure what to do about it. She wrote that she would sort it out once he had finished that book.
He then had a reading session with the class teacher who sent him to get another stage 15 or whatever he wanted as he was clearly managing well, and enjoying them, and they are interesting.
He finished it fairly quickly and took it in today, whereupon he was handed a stage 12 by the TA and told she had chosen it for him.
Why would she want him to be back on stage 12? This is the bit I don't understand - surely he needs the challenge of books that are a bit harder than this? If he is coping well with 15 then why on earth put him back on 12?
I'm not pushy but he is my first able reader and so I'm a bit stuck as to how to handle this one without seeming tw*ttish 
Any thoughts welcome.