Fowler's Modern English Usage endorses "different from," "different than," and "different to," pointing to examples of each cited in the OED dating back to 1590, 1644, and 1526 respectively. I think, however, that "different to" is apt to sound strange to most native speakers of American English. Preceding a clause, I'd use "different than." In your example, the most natural sounding option (to me) is: "... the classrooms and backyard looked completely different than when I was a little boy," or perhaps "... the classrooms and backyard looked completely different than I remember them from my childhood."