Where Did I Come From and What's Happening To Me are still really popular. There are also two others that schools were loving when I was selling books "Secret Girls' Business" and "Secret Boys Business". There was also a follow up to the girl version. They were aimed at around grade 5 to year 7 or so.

ETA - Sorry, Onyx, just reread your post and saw the ages you were after. The first two are better for the age you're looking for.