Great discussion. These Japanese high school athletes are as advertised - no fakery, drugs or over-age tricks. Times are FAT on the track (I've attended many meets). But their 5k times are generally 30-60 seconds faster than their street 5k times in an ekiden, despite the fact that the Kyoto ekiden street race is a fast course (I was there watching), so the differences (track, street, cross country) should be part of the discussion.
I also think that what produces these truly great marks for thousands of Japanese high school athletes is different from what produces world class performances among elite 20 something Americans. (Caveat - drawing 20-30 elite high schoolers to a magnet school seems to have a multiplier effect, similar to American collegiate training)
For most of these Japanese kids, high school is the end of their competitive career. A small percentage of Japanese boys get to go on to collegiate training (shifting away from track and focusing on 10k to half-marathon) and only a micro percentage of girls get a similar opportunity. The generous opportunities for anyone in American colleges who wants to continue competing in track/xc turns the whole equation upside down.
I would also wager that a surprisingly large percentage of American high school runners also play basketball, baseball, soccer, field hockey, etc. and/or are involved in performing arts and/or are laudable scholars. Not even thinkable in Japan.
And let's not the undervalue of education (shock). If we ranked the US and Japanese high schooler runners (and I have had many of the latter in my Japanese college classroom for 20+ years) based on who goes on to be doctors, lawyers, engineers, entrepreneurs, executives and community leaders, I would bet there would be complete reversal.