I dont deny the occurence of promiscuity in High school, but it is nowhere near the frequency ocurring in college, and I don't think it is particularly higher than it was 10 yrs ago (I have/had 3 teenage girls, so I know of what I speak, to some extent)
"Group dating" was actually pretty big in the 30's and 40's, and IMO the key indice theyre trying to track here is promiscuity, which is totally out of control at the college level, from what I can tell. At the high school level, the trends are not as clear.( In order to "hook up", you need a certain level of personal freedom, more easily obtained at college. )