A barrel bomb killed at least seven civilians, including a teacher and four children, when it hit a school in Syria's northern city of Aleppo on Sunday, a monitor said.
"The primary school students were playing in the courtyard when the barrel bomb hit," said Rami Abdel Rahman, who heads the Britain-based Syrian Observatory for Human Rights.
He said the school is in Saif al-Dawla, a district where control is split between the regime and opposition.
"The children were taking a test there today because they were afraid their own school would be bombarded," Abdel Rahman said.
The centre's director and other civilians are in critical condition after the attack, he added.
An AFP correspondent described buildings in ruins and small bulldozers working to clear the rubble.
Classroom walls had completely collapsed, exposing mangled chairs and desks, and men carried whatever books they could save out of the building.
Barrel bombs are crude weapons made of old barrels or containers that are packed with explosives and typically dropped from helicopters.
The weapons are often used on rebel-held neighbourhoods in Aleppo, once Syria's commercial hub but now divided between regime and opposition control since fighting broke out there in 2012.
Just last month, more than 100 schools in the city had to close temporarily after a regime air attack killed five children and three teachers.