I was a state champ and D1 runner at P5 school. My high school progression was about 30 miles per week freshman cross country season, got up to 40 that year in track. Then increased it about 10 miles per year, was a consistent 60 miles per week senior year topping out at 70 miles with no days off.
The most I heard of people was about 90 miles max in high school for some high mileage Illinois runners from schools like Palatine/Nequa/Elmhurst.
I'd stay in this rance, do at least 30 the freshman year, 45 tops, and don't build above 80 by senior year but build up to at least 60. It does also depend on the intensity of the workouts. If it seems like they're racing all the time and workouts are really intense lower mileage might be better to avoid injury. But if workouts are controlled and not time trials, then they can handle more mileage. I would prefer the latter, more mileage and appropriate workout efforts.
Be sure to regularly do core workouts, this stuff goes a long ways and can be done every day.