Lake said the school compound was "repeatedly attacked," adding that it may be the deadliest attack on a school since the war began more than five years ago.
"When will the world's revulsion at such barbarity be matched by insistence that this must stop?" added the UNICEF director.
Asked about the attack, Russia's UN ambassador Vitaly Churkin responded: "It's horrible, horrible. I hope we were not involved."
Syrian government forces and their Russian ally have been accused by rights groups of carrying out indiscriminate attacks on civilian infrastructure.
The White Helmets civil defence group released pictures of four rescue workers clambering over a mound of rubble in search of survivors after what it said was a "double-tap" strike on the school.
The raids hit Hass around 11:30 am (0830 GMT), an activist with the opposition Idlib Media Centre told AFP.
"One rocket hit the entrance of the school as students were leaving to go home, after the school administration decided to end classes for the day because of the raids," the activist said, speaking on condition of anonymity.
Other activists from the province circulated a photograph on social media of a child's arm, seared off above the elbow, still clutching the strap of a dusty black rucksack.
Shaky video footage depicted rescue workers sprinting towards the site of the raids and pulling a frail, elderly man out of a collapsed building.
The authenticity of the pictures and footage could not be independently verified.
The latest attacks took to 89 the number of civilians killed in air strikes on Idlib province in the past seven days, said the Observatory.
-AFPÂ
