Live updates: Follow the latest news on Israel-Gaza
Philippe Lazzarini, head of the UNRWA, the world body's agency for Palestinian refugees, denounced the strike after viewing “horrifying images and footage of scores of people killed and injured” at one of the agency's schools “sheltering thousands of displaced”.
Gaza's Hamas-run Health Ministry said at least 50 people were killed at the school in the Jabalia refugee camp, and that 32 more people, all members of a single family, died in a second strike on the camp.
“These attacks cannot become commonplace, they must stop,” Mr Lazzarini wrote on social media.
The Israeli army did not specifically comment on the strikes, but said its troops were expanding operations in the Gaza Strip, including parts of Jabalia, to “target terrorists and strike Hamas infrastructure”.
“The IDF is continuing and expanding its operational activities in Gaza,” the military posted on social media.
“Over the past day, our forces conducted activities in the Zaytun and Jabaliya areas, during which they encountered terrorists who intentionally operated from civilian areas and attacked the troops using anti-tank missiles and explosives.”
The UNRWA confirmed a strike had hit the Al Fakhoura School in the Jabalia camp.
Its spokesman Adnan Abu Hasna said: “Right now we don't have UNRWA official figures, but this is credible information.”
He added that there had been two rocket strikes in the area.
“These attacks must stop and we should establish a humanitarian pause immediately,” he said.
“We provide the Israelis – twice a day – all the GPS of all our installations. Especially those which shelter internally displaced people, like Fakhoura.
“This school was sheltering thousands of displaced, we don't know why they attacked it.”
Videos posted on social media appeared to show bodies covered in blood and dust on the floor of a building.
Jabalia is the biggest refugee camp in Gaza, where about 1.6 million have been displaced by more than six weeks of fighting between Israel and Hamas.