Actually I think that you have done very well throughout.
It's nice see folk particpate in the Forum both asking and providing.
My creed is that "today is the first day of the rest of my life" and life is just one long learning curve. I'd be sad if I didn't learn something new each day.
Ken Anderson - The Lulu'ers Professor
lol. Just having a discussion about the use of that category name in the US and UK on another forum - and how well-known it was. It's basically 8-12. Though I prefer to write for the 9-14 range (which doesn't appear to have a name), and there are variations between the UK and US.
Middle Grade readers are confident readers, so no need to hold back on sentence construction, vocabulary, etc. then there's teen (which I think the US miss out and just go straight for YA), then YA...it's not a precise science. 
Oh right, it's a school book then? 
It's hard to categorise books over a certain age. Potter was a 'children's' book, so was The Hobbit, so are some of Terry Pratchett's books, and all are enjoyed by many ages.
It's also not easy to tell just how well a kid over five can read. It depends on the kid. It also depends on the adult on how well they can read! It's not clear-cut.
Often I think if a book's main characters are children, then it may be a kid's book. But even that is not always the case!
There's also that 'sex' and swearing thing.
If it contains sex it may have to be 'over 16', but in some US States that would be over 18 or 21! It may even be 'porn', so, well, may be banned! Even a kiss may seem OTT to some people!
Swearing. Debatable nowadays, but the C and F word would make it over 16, possibly even 18. The odd "bloody hell" may be OK for the over 8s!
Some publishers would say that only an adult (whatever that is) can understand 'long' words. (Others should look them up then).
Basically, if it is not obviously Spot The Dog, or full of swearing, sex and gore, I would leave it for the reader or even parent to decide, or even teacher or whatever.
But then again, there are some minorities who will object to anything. (Just don't mention Darwen ... 
)
