I'll bite...
NEPSTA stands for the New England Private School Track Association which I tend to think of as consisting primarily of the schools in the eastern part of Massachusetts-- which is a combination of day (re: BB&N, St. Sebastian, Roxbury Latin, Thayer) and boarding schools (Tabor, Milton Academy, St. Paul's. These schools are part of a league called the ISL in which all sixteen field fall XC teams, but less than ten field spring track teams and as a result, the ISL teams that do created NEPSTA and incorporated smaller day and boarding schools like Pingree, Tabor and Windsor to form NEPSTA) They participate in a regional league called NEPSAC (New England Private School Athletic Conference) which is made up of NEPSTA and the well known, national in scope boarding schools of Phillips Andover, Phillips Exeter, Deerfield, and Taft to name a few.
They are their own animal and are not part of any high school federation. Many of the schools outside of NEPSTA have a formal post graduate program...a 5th year. The NEPSTA schools do not although many hockey players transfer from public school and repeat as juniors and seniors in these schools so while they don't have a formal post graduate program, one could say its "informal."
All the schools have good to great academics, facilities are top notch (inclusive of on campus hockey rinks) but if your question is about track and field and XC....
The league's philosophy most closely resembles the NESCAC schools (re: Tufts, Williams, Amherst) and to a lessor extent the Ivy league.
Most of the male distance and middle distance athletes fit very well in D3 programs. Times on the track and in XC would not be considered elite and your rarely, if ever, see these kids compete outside of their leagues. Once the season is over, the season is over as the majority of the schools do not have indoor track, or the better schools outdoor track, and as most have mandatory athletic participation rules, the athletes move on to another sport. So they are traditionally "under trained" and maybe, my view at least, "over raced" as they race weekly in the fall at the 5K distance and have a very short spring track season in which league meets and championships are all fit in a period from early April to mid-may. I thinking coaching is "spotty" through out the schools as you have some very good coaches, and some very average ones (so its not much different than public schools in that regard).
Because of the presence of boarding schools, the coaches in the league have a gentleman's agreement relative to off season contact, particularly over the summer. As your athletes are far flung and not located in the same town or city like your public school, few train seriously over the summer and I think the training and racing philosophy is more tied to getting the majority of kids fit than it is to produce good runners. Also, the mandatory athletic participation rule creates huge XC and T&F teams as these sports take everyone of every ability.
There are some D1 girls who went to NEPSAC schools racing at a very high level. Kayla Hatton at Stanford went to Exeter...but she didn't race for them her senior year, and due to injury only ran for her school her freshman year. Emily Stark of Yale, a freshman, went to Middlesex and is the #2 or 3 girl at Yale this fall.