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, according to the AFP news agency.

“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, according to AFP.

The center’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.

Aleppo has been the target of “barrel bomb” attacks, which the regime constantly uses against rebel held areas of the city.

Government forces frequently bomb rebel-held neighborhoods, and the rebels often retaliate with rocket and mortar fire. Both sides have been criticized by human rights groups for firing indiscriminately on civilians.

Just last month, more than 100 schools in Aleppo had to close temporarily after a regime air attack killed five children and three teachers.