USNWR has specialty ranking for non-profit. The social entrepreneur ranking is pretty close to that (maybe cross reference it with the entrepreneurship ranking). Someone already mentioned the Aspen institute's ranking, but that ranking is gear toward the sustainability aspect, which is only a subset of "social entrepreneurship". You can also look at the schools with gold Net Impact clubs to get an idea of how strong it is in that area.
The non-profit ranking for reference:
1 Yale University New Haven, CT
2 University of California--Berkeley (Haas) Berkeley, CA
3 Stanford University Stanford, CA
4 Northwestern University (Kellogg) Evanston, IL
5 Harvard University Boston, MA
6 University of Michigan--Ann Arbor (Ross) Ann Arbor, MI
7 Duke University (Fuqua) Durham, NC
8 University of Pennsylvania (Wharton) Philadelphia, PA
9 Columbia University New York, NY
10 New York University (Stern) New York, NY