Parents arriving to the school said the building sustained damage.
Two firefighters sustained minor injuries battling the blaze.
Authorities were investigating if vandals broke into the school and set the building on fire.
The small private school, which serves K-12 students, is located in a large house in Flatbush.
It uses the "Waldorf and Montessori education principles," and most classes have no more than a maximum of 18 students.
NEW YORK AND TRI-STATE AREA NEWS