It is definitely possible to be a successful High school athlete while running under 30mpw year round. However, it really depends on the individual if this approach works. It is possible that an entire team can perform well with low mileage, but that doesn't mean that some kids would've run better doing a bit more base work.
I never went to High School in the U.S., but in my training group we had a few high school aged guys that ran around 15:30 for the 5K. Some ran 40-50 miles a week, but others significantly less than that. I ran about 25 miles a week in my senior year while running 15:28min (5K was an off-distance for me). After finishing High school I went to college in the US and tried to do Higher mileage but was injured 90% of the time. I eventually only ran well in the summers when I would go home and go back to my original training routine (25-30miles a week). I ended up running 3:48 for the 1500m in my sophomore year.
I think a good coach should recognize if someone is better off doing high or low mileage. I really like the team-oriented aspect of running in the US, but sometimes it is taken too far. It is so easy to individualize people's schedule a bit while allowing them to train with the group (letting them do a shorter long run, let them run less reps, let them do 1200m reps instead of 1600, etc.) and I don't understand why coaches don't do this more often.
I don't understand why some coaches claim to be a high or a low-mileage coach or schools claim to be a high or low mileage program. Is it because they believe this system should work for everyone or do they think it is too much effort to individualize their approach?