Basically you didn't bother to read my posts, or you have some serious issues with reading comprehension.
My principle is that it is better to slightly overtrain than to significantly undertrain. Also, most HS students and coaches have no idea where that line between optimal training load and overtraining is, so being risk-adverse, they tend to play it way to safe and end up significantly undertraining. Most HS coaches tend to develop training programs for the lowest common denominator, and are exceptionally risk-adverse because they don't want to risk any injury (liability issues). In addition, most HS programs emphasis the "team" over the "individual", and therefore the goal is to get the Varsity boys running together in training all the time, which means finding the lowest common denominator once again. Therefore most HS training is nowhere near what a talented HS kid can physically handle.
Also, these training resoures and online coaches, as good as they are, also play it conservatively. I have had discussions when I was running in college with some of these certified coaches for various programs, and when I asked them whether my NCAA D1 coach was crazy for not following them, they basically all say, "well you guys are different, you guys can handle a lot more stress and adapt faster than the typical runner." So the question is, can we handle more because we were born this way, or because we trained this way for a long time? Probably a bit of both.
Too long didn't read version: there is a strong and direct correlation between high school mileage and high school 5K times. I have never seen a HS kid run 60+ mpw and fail to break 17:00 in the 5K, have you? I have seen a horde of HS kids run 25-35mpw and run anywhere from the 18s-22s, in fact that probably speaks for the majority of all HS XC runners. A few talented kids who run 25-35mpw end up in the 16s, but they are a minority. The kids in the 15s are well into the 60s or above.