Syria regime-backed airstrikes pummeled a school complex in the town of Hass, killing at least 35 people, including 11 children, according to CNN.
The Syrian Observatory for Human Rights told CNN that the death toll is expected to rise.
Six teachers were among the dead, according to UNICEF.
"This is a tragedy. It is an outrage. And if deliberate, it is a war crime," UNICEF executive director Anthony Lake said in a statement. "This latest atrocity may be the deadliest attack on a school since the war began more than five years ago."
Although the human right's organization is placing blaming on the Syrian regime for this latest round of deadly airstrikes, "It is difficult to distinguish between Syrian jets and Russian jets," CNN noted.
In September, Russian-backed airstrikes killed more than 100 people in Aleppo.
Nearly 500,000 people have been killed in Syria since the war began in 2011.