Your body will adapt to stress if it's incremental and combined with sufficient opportunity to adapt. A kid who ran 20 mpw all fall is likely to get injured if they start running 70 mpw this week after taking 6 weeks off. However, a kid running 60 mpw last fall who continued running base mileage and increased ~2 mpw isn't likely to have a problem.
A huge problem is kids that run 35 mpw in high school then jump to 70-80+ mpw in the fall of their freshmen year at college. If you plan to run in college, find out what kind of mileage you will be expected to run next fall and develop a plan to reach that without any large jumps.
A conservative rule of thumb is to increase 10 mpw per year. This could look like:
9th 30 mpw
10th 40 mpw
11th 50 mpw
12th 60 mpw
Fr 70 mpw
So 80 mpw
Jr 90 mpw
Sr 100 mpw
5th yr Sr 110 mpw
I might be concerned about a high school athlete averaging 90 mpw without knowing how they got there. If they were a 30 mpw athlete for 3.5 years and then got super motivated after watching "Pre" and then started running 90 mpw, I would definitely be concerned. But, if they were from an active family and were running 50 mpw as a 9th grader, I would have no physical concerns about 90 mpw as a 12th grader.
Focus on progressively and incrementally increasing the stresses imposed, not on any absolute number.