A school shapes your short-term goal much more than your long-term goal, especially when it’s entrepreneurship. (So many things on industry and entrepreneurship will become dated by the time you decide to pursue it in the long-term.) If your long-term goal is social entrepreneurship, I think it’s better to look for schools which are solid in entrepreneurship (and seeing the interest of applicants, most schools have beefed up their entrepreneurship offering in the past few years) than super-niching it to social entrepreneurship.
Second, if you join a non-profit immediately after your MBA and work there for certain number of years, some schools offer loan forgiveness. You need to check the financial aid page on the website of a particular school or Google ‘loan forgiveness’ + [school name + MBA] to see if they offer it or not.