A health ministry official in the Gaza Strip said more than 80 people were killed on Saturday in twin Israeli strikes on the Jabalia refugee camp.
"At least 50 people" were killed in a dawn strike on the UN-run Al-Fakhoura school in the camp, the ministry official told AFP, sparking condemnation from the United Nations' humanitarian chief.
UN officials said thousands of displaced Palestinians had been sheltering at the school and Martin Griffiths denounced the "tragic news of the children, women and men killed."
"Shelters are a place for safety," he posted on X, formerly Twitter. "Schools are a place for learning.
"Civilians cannot and should not have to bear this any longer. Humanity needs to prevail."
Other UN officials have also expressed their shock at the strikes, including UNRWA Commissioner-General Philippe Lazzarini, who said he had seen "horrifying images" of the attack, adding that "a humanitarian ceasefire cannot wait any longer."
Social media videos - verified by AFP - showed bodies covered in blood and dust on the floor of a building, where mattresses had been wedged under school tables.
Jabalia, in the northern Gaza Strip, is the biggest refugee camp in the Palestinian territory, where some 1.6 million have been displaced by more than six weeks of fighting between Israel and Hamas.
Earlier this month, the health ministry said 15 people were killed by an Israeli strike at the same school. UNRWA said four of its schools in the Gaza Strip housing people displaced by the war had been damaged by bombings.
A separate strike on another building in the Jabalia camp on Saturday killed 32 people from the same family, 19 of them children, the health ministry official said.
The ministry released a list of 32 members of the Abu Habal family it said had died.
At the start of November, more than 200 people had been killed and hundreds more wounded in three straight days of Israeli bombardment of Jabalia.
Israel has vowed to destroy Hamas in response to the 7 October attacks which Israeli officials say killed about 1,200 people, most of them civilians, and saw about 240 people taken hostage.
The army's relentless air and ground campaign has killed 12,300 people, more than 5,000 of them children, according to the Gaz's health authorities.