We have come across a similar problem to yours. Our daughter is 9 years old and she is also the youngest in her class. She is not at the bottom of the class, but she is also not excelling academically either. We struggled with the decision to send her to school at such a young age, as the trend seems to be now for keeping children back a year if they are born close to the cutoff. I wish now in hindsight that we had kept her back and not sent her early. Now as the time approaches (rather fastly) for her to be starting high school it is very obvious to us that she is in no way ready in regards to maturity. After much thought and talking to her teachers on their thoughts, we have decided that she will repeat next year at a new school, as we feel it is too cruel for the kids to repeat at the same school as the chances of cruel teasing is very high.
One thing that I have found with parenting is that sometimes we make decisions that take care of the immediate and we don't consider the longterm effects of that decision. If only I had considered how young she would be going into high school I would of definitely started her latter rather than sooner.