What is a low IQ score for a 11 year old girl?

IQ tests are designed so that the average score for every age is 100. Your IQ does not specifically depend on your age - it doesn't necessarily get higher as you get older!

Roughly 68% of people have IQs between 85 and 115. That would be perfectly normal. Children with scores between 70 and 85 sometimes need special services to be successful at school. Anything below 70 would be a sign of being impaired in some way, and would definitely need the support of resource or special ed. teachers to have an appropriate school experience.