The death toll is expected to rise as dozens of others were injured in the Wednesday attacks, said Majd Khalaf, a communications officer with the rescue group, also known as the White Helmets. Another rescuer told the activist-run SMART News Agency that around 50 children were leaving the front door when the strikes hit.
The Syrian Observatory for Human Rights blamed the strikes on the Syrian regime and Russia, but it is generally difficult to distinguish between Syrian and Russian jets.
Russia has been supporting President Bashar al-Assad's regime with airstrikes since September 2015, and the allies have been widely criticized for targeting civilians, and hospitals and schools in rebel-held areas.
Jets hit the complex and the surrounding area in the village of Hass, in Idlib province, six times and many people remain in critical condition, Rami Abdulrahman, director of the Observatory told CNN. The school had students from the first to ninth grade enrolled, he said.
The White Helmets said that there were three schools in the complex, and shared photos on social media showing buildings reduced to rubble.
UNICEF, the UN agency for children, said that the strikes may amount to a war crime.
"It's a tragedy. It's an outrage. And if deliberate, it's a war crime" UNICEF executive director Tony 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," he said.
"When will the world's revulsion at such barbarity be matched by insistence that this must stop?"
The Syrian Civil Defense -- a volunteer rescue group also known as the White Helmets -- said that there were three schools in the complex, and shared photos on social media showing buildings reduced to rubble.
Many hospitals and schools in Syria have been forced underground to avoid becoming targets of regime and Russian airstrikes, particularly in rebel-held eastern Aleppo.
A UN report in August
found "sufficient evidence" that the regime had used chemical weapons on civilians, particularly barrel bombs laden with chlorine gas in Aleppo. The Syrian government and Russia both deny the accusations.
'Next generation is a lost generation'
The school strikes come as pressure mounts on the international community to find a solution to the war, even though talks are faltering between the US and Russia, major players in the conflict.
Attention in the fight against ISIS has also been diverted to Iraq in the past two weeks as a major operation got underway, involving a coalition of more than 100,000 people, to seize the city of Mosul from ISIS control.
Stephen O'Brien, the UN under-secretary-general for humanitarian affairs, made a desperate plea to the UN Security Council Wednesday in New York for progress on a solution.
You can listen to part of the speech here:
He highlighted how children had been caught in the conflict's crosshairs, especially in the city of Aleppo.
"Children in besieged eastern Aleppo were due to resume school in late September. They didn't. Instead, shell-shocked children are retrieved from rubble and left writhing in bloody clothes on dirty hospital floors," he said.
He said that across the country, one in four schools had shut down, more than 52,000 teachers had left their jobs and over 2 million children remained out of school. "And another 400,000 are at risk of dropping out as the horrors of this brutal and savage war continue unabated," he said.
"What future do these children have -- illiterate, orphaned, starved and maimed? What future does a country have when its next generation is a lost generation? These children do not have the luxury of waiting for another Geneva, Vienna or Lausanne to succeed. They need our protection now."
US Defense: Offensive against ISIS in Raqqa 'within weeks'
Despite the enormous offensive in Mosul, to which the US has dedicated hundreds of special forces, US Defense Secretary Ash Carter said Wednesday it would only be a matter of weeks before Washington and its allies were ready to drive ISIS from the Syrian city of Raqqa, the militant group's symbolic capital.
The preparations come amid fears ISIS is plotting an attack somewhere around the world from inside the city.
The plan "has us generating those forces in a matter of weeks ... generating them and positioning them for the isolation of Raqqa," Carter said, without giving any further details.
"That's what I'm going to say," he said, adding, "And not many weeks."
It has been "long planned" that the US-backed assault on Mosul -- which started earlier this month -- and Raqqa would be overlapping, Carter said.
Turkish President Recep Tayyip Erdogan said Thursday that he had spoken to US President Barack Obama in a phone call to express his country's willingness to be part of the Raqqa offensive.
"Last night we had a long conversation with Mr. Obama and told him about the steps that we will take," Erdogan said.
"I said 'Let's take Raqqa back from Daesh together. We will handle this together with you. We have the power.'"
Turkey is trying to take a bigger role in operations on its doorstep, vowing to participate in the current offensive to retake Mosul, despite Iraqi leaders saying Ankara was not needed.
He also said that he told Obama there was no need to use the Kurdish militia groups, some of which the US supports but Turkey considers terrorists. Some Kurdish groups, such as the Peshmerga, are at the forefront of the Mosul operation.