After spending the last 11 years holding classes in hallways, a parking lot, or from a cart, one of the first charter public schools in Massachusetts finally has a home big enough for its 375 students. With some creative financing, aggressive fund-raising, and strong community support, the Francis W. Parker Charter Essential School has bought a former elementary school and a 13-classroom modular building at Devens. Students and faculty moved into the new space this month. The Parker school started out in a windowless building at the former Fort Devens Army base. Eight years ago, it moved to the former elementary school on the base, even though officials knew it was too small.