I'm concerned my child is shorter than her friends.

See your doctor. Your pediatrician will plot your child's height and weight during each well child exam. Ask how your child's height compares to her same age peers, if her growth velocity is normal, and what stage of puberty that she's in. These answers will determine if she should be referred to an endocrinologist, or if she is growing normally.
May be normal. A child's size depends on many factors but there is a wide range of normal. There are tables that show how your child compares to others of the same age and how he compares to himself, according to what he has grown in the past. It's important to understand this and your child's doctor should be able to give a clear explanation.
Differences. Every kid is different... If three different sized adults walked down the street, no one points and worries. Just remember that all kids have different growth potentials. In addition, each kid starts puberty at a different time and thus growth spurts at a different time, making ages 9-15 a wired time of crazy growth patterns. Talk it over with your doc and look at your child's growth curve.