Brief forays into high mileage are actually good for a developing teenage runner. OTOH, staying high all the time will often produce the best results at the high school level, but this may come at a small expense later in life.
This isn't to discourage a high schooler from going high on occasion. 120 miles in a single week (barring injury) is almost certainly going to help a teenager who is already accustomed to 17-mile days in shorter blocks (2-5 days) or who has done a 105-mile week in the past.
High school runners are generally light and recover quickly (and boys have a surplus of testosterone at that age, which makes it easier to handle a heavy workload), so it might feel easy for awhile to keep the big mileage weeks rolling. But it's best for future development to have a short excursion into high mileage, followed by a very low segment, then a return to normal mileage levels.
What are good "normal" levels for a high school Senior? Probably around 70-75 per week for boys and 55-60 for girls (provided this level has been attained safely during previous seasons).
It also depends on what the goals are. Many runners have no aspiration to compete in college or beyond in more than a recreational capacity. For these runners, maxing out in high school might not be that bad, and being part of a state (or national) championship team would unquestionably be the highlight of most people's careers. Schools like York and The Woodlands come to mind as schools with deep, rich traditions of championships. These guys do a lot of miles, which might cost a little for the future. But if you were an average high school runner with less-than-stellar talent and no aspirations for world records, wouldn't you want to make the elite seven at York and be a part of a coveted championship?
And guess what.
Maybe that "average" high school guy will go farther with what he thought was modest talent as a result of his early success, disciplined habits, and the inspiration gained from his state championship than he would have gone by settling for mediocrity.
Surely there are African runners who do too much too early in life (or do it too hard), just as there are Westerners who meet the same fate. We never hear about them. But, in general, the serious runners who bank the big miles are the ones who will shine when the final results are in. To prepare for high mileage, push the boundaries out with short blocks of a few high days, then gradually progress to high weeks, then to high months. The high school years are not too soon to begin this process.