"A girl running 18min is ok, but it is not pretty good. She's almost half a mile behind the average top-ranked high school girl."
That's your exact quote. The fact that it says 18 mins is even more ridiculous, so I'll just give you the benefit of the doubt and assume you meant 18.30.
So, you're saying this mythical top ranked high school will finish the race as our 18.30 girl is half a mile in arrears. Assuming even pace, she ('slow' girl) will pass through 4200m in 'about' 15.30. If we assume you actually meant 18 minutes in the above quote, that time goes down to a ridiculous 15.05(ish). Put simply - she would not be half a mile behind, or even 'about' half a mile (whatever that means anyway)