The dead from Wednesday's airstrikes near a school complex in northwestern Syria included 22 children, the head of the UN Children's Fund (UNICEF) has said.
UNICEF Director Alan Lane said Wednesday that six teachers also died in the attacks, while the Syrian Observatory for Human Rights said that 35 people in total had died.
"This is a tragedy. It is an outrage. And if deliberate, it is a war crime," Lane said in a statement.
"This latest atrocity may be the deadliest attack on a school since the war began more than five years ago."
The Syrian Observatory for Human Rights said that at least six air raids by suspected Russian jets hit Haas village in Idlib province.
"The dead children are schoolchildren and the planes are believed to be Russian," Rami Abdel-Rahman, the organization's head, told dpa.
Activists posted pictures of bodies of small children and schoolbags covered with blood allegedly as a result of the bombing.
In recent months, Idlib, a hub for rebels, has been the target of air bombardment.
The Observatory said that 89 civilians, including Wednesday's toll, had been killed in airstrikes mounted by the Syrian regime and allied Russia in Idlib over the past week.
Idlib is controlled by an alliance of opposition groups and hardline jihadists including Jabhat Fatah al-Sham, an al-Qaeda-linked group previously known as the al-Nusra Front.
In his statement, Lane described Wednesday's attack on the school as "one more scar on Syria's future."
"When will the world's revulsion at such barbarity be matched by insistence that this must stop?" he said.