T.K.
She sounds like my youngest, who is now 13, and still a very introverted child.....just like her 45 yr old mother. Her brother and sister are very outgoing, never met a stranger types, and so a lot of people see the youngest one as having a "problem." Unless/until it creates a problem for her, actually stopping her from doing things she WANTS to do, I wouldn't worry about her, especially at her age.
If you're still concerned, though, you could try talking to the school psychologist about her....and my oldest daughter has a good counselor at Integrated Youth Services in Beavercreek.