Many schools use google for email, calendars, and chat. It's google for business, but I believe it's free (or relatively cheap) for education institutions. Theoretically all those users would be using Google and perhaps Google+ with enough encouragement.

Many other schools choose MS Live and those students would not necessarily have any reason to use Google+. Interestingly, at least at UMass Boston, the primary justification for MS Live was because MS claims it can identify the physical machines your data is housed on, whereas Google does not make such promises. This is only important for checking off boxes when audited.