How to make 14 year old girl go back to normal school?

My friends 14 year old sister stopped going to normal school in 8th grade and decided to do online school. It is time for her to go to high school now and she still wants to do online school. She hasn't grown up and is still stuck in the child mind set, so she acts like a 7th grader. She only talks to a few friends her own age. Any ideas on how to convince her to go to a normal high school? At least to try it out for a month or so?