Sure, natural maturation will usually lead to improvements no matter what kind of training you're doing as a young athlete. But if that's your argument against high mileage then we might as well say, "Heck. Just don't train at all. You'll improve through natural maturation!"
Of course that's not the best training philosophy. Natural growth and strength increase can often lead to improved athletic performance, but this PLUS hard training will reap the greatest rewards. Especially if a runner is interested in the types of performances that will lead to a scholarship, or at least being walk-on worthy. Sure there are some great talents out there who can break school records on 15 mpw, but that's not typical.
Moderately high mileage (50-60 mpw) is ok, even for high schoolers. Just be smart about it.