Allison Keil and Sara Stone, young and idealistic teachers, founded the Community Roots Charter School in Fort Greene, Brooklyn, six years ago. The school has 300 students in kindergarten through fifth grade and, this year, a waiting list of 800, the New York Times reports. Last spring, the school delayed its plans to expand into a middle school, amid vigorous community opposition because Community Roots would have taken more space in the building it shares with two other schools…