6:15 in 7th grade is pretty good, but as the other posters have said, it is WAY too early to tell. if she hasn't hit puberty yet, that can throw a wrench in things. if she doesn't have the drive to put in the training that running really fast will require, that will cut her career short. if she can't handle much training because of injury, she might not reach her potential, etc. etc. etc.
just to give you an example, i ran 5:44 in the 7th grade (i am female). i finished third in my school district. the girl who won ran about 5:35, ran very well her freshman and sophomore year in high school and then had quit altogether by her senior year. the girl who finished 2nd ran low 5:40s, had a decent high school career but nothing special, and ended up deciding to play DI soccer (her true passion). i broke 5 in high school (the only one of the three of us to do so), but never got faster than about 4:45 in college.
meanwhile, i had a high school teammate who took up running in high school and only started training seriously around her junior year when she had to quit soccer because of an injury. she never won a state title in high school, but she now runs professionally and has run in the 15:30s for 5k.
hard to tell where any 7th grade girl will end up in this sport. whatever you do to encourage her, make sure you keep the sport fun at this point. she'll stick with it if she enjoys it... and if she sticks with it, who knows what might happen?