BEIRUT, Lebanon — Organizers had worked until dawn hanging children’s artwork on the wall of an elementary school in Syria’s largest city: a bright green tank under a round yellow sun, a girl in pigtails defying a soldier’s bullets, a missile plunging from a warplane toward the school building.

But hours before the exhibit was to open on Wednesday, creating a rare space for children’s creativity in a ravaged district of the northern city of Aleppo, Syrian government aircraft bombarded the school, residents and anti-government activists said. At least 20 people, including 17 students and two teachers, were killed, they said, and many were wounded, including the school principal and the local artist who mounted the exhibit.

“She asked each one to draw what he dreams of,” said the distraught sister of the organizer, a painter and volunteer with a psychological support group called Fingerprints of Hope. The children’s art, she had written on the exhibit’s flier, reflected not only “blood and pain,” but also perseverance and hope that “stands as a blockade in front of death.”

The attack was one of scores of aerial bombardments, including at least 85 barrel bombs, explosives-filled barrels dropped from helicopters that the government has unleashed on insurgent-held areas of Aleppo since the United Nations Security Council called in February for all parties to end such indiscriminate bombings. Hundreds have died.