5K improvement--high school girls

Question
I have a 15 year old daughter who plays high school tennis, she recently ran her first road race of her life, a local 5K with a time of 21:45.  Cross-country and tennis compete at the same time at our local high school and she cannot do both.   She can run road races on some weekends.
Any suggestions on training, and what type of time improvements do girls typically achieve from their sophomore to senior years of high school?

Answer
A 'typical' improvement doesn't exist, really.  Why? Because there are too many factors involved with improving an athlete who plays multiple sports.  My suggestion is that if she is focusing on tennis, a 5K improvement shouldn't be the main goal, improving in tennis should be.  She can do a 5K for fun, and let it be at that.

Rick Karboviak