Hamas, which controls Gaza, says it will not stop fighting until the blockade, maintained by both Israel and Egypt, is lifted.
The current conflict is now the longest between Israel and militants from Gaza.
A 2012 offensive lasted for eight days, and the 2008 conflict went on for 22 days.
Children were said to be among the wounded in the school strike
Walls were wrecked by the shelling, according to witnesses
Israel sent in ground forces after 10 days of air assaults to stop rocket and tunnel attacksMr Gunness from the UN Relief and Works Agency (UNRWA) told the BBC that Israel had been told 17 times that the school in the Jabaliya refugee camp was housing the displaced.
"The last time was hours before the fatal attack," he said.
"Our initial assessment is that it was Israeli artillery that hit our school."
Chris Gunness: "Children were killed as they slept next to their parents"
He said there were "multiple deaths" including women and children, adding that the attack caused "universal shame".
Images from the school showed large holes in the walls and roof.
Bob Turner, UNRWA's Gaza director, said the UN was "confident" Israel was responsible.
He said UN workers had collected fragments of projectiles that suggested they were artillery shells fired from Israeli positions to the north-east of the school.
The BBC's Orla Guerin was given access by the Israeli military to a tunnel they say was used by Palestinian militants
The Israeli military said the incident was under review.
It said in a statement that its "initial inquiry suggests that militants fired mortars earlier this morning from the vicinity of UNRWA school in Jabaliya".
It said soldiers "responded by firing towards the origin of fire".
The military later said a ceasefire would be in force between 15:00 (12:00 GMT) and 19:00.
Polls suggest almost unanimous support in Israel for the military operation
Mosques, power stations and many residential buildings have all been hit in the Israeli bombardmentHowever, it would only apply to areas where Israeli soldiers were not currently operating and residents were warned not to return to areas they had previously been asked to evacuate.
Lt Col Peter Lerner of the IDF told the BBC: "I hope that Hamas will hold their fire as well, because otherwise things are going to get messier."
Israel has repeatedly accused Hamas of using schools and civilian areas as bases to launch attacks.
Last week, another UN-run school was hit, with Palestinians saying at least 15 people were killed.
But the Israeli military denied the killings, saying a single "errant" shell had landed in an empty courtyard.
In other developments:
- The UN on Tuesday revealed that a cache of rockets had been found at one of its schools in Gaza - the third case of its kind - and condemned it as a "yet another flagrant violation of the neutrality of our premises"
- A monthly opinion poll of about 600 Israeli Jews by Tel Aviv University suggests 97% support the current military operation
- A baby who was born after her mother was killed in Gaza, making headlines around the world, has died
Israel stepped up the intensity of its strikes on Tuesday and overnight into Wednesday, saying it had hit a number of tunnels dug by militants to attack Israel.
But the Israeli military said rockets continued to land in Israel from Gaza.
Palestinian officials said Gaza's port had been destroyed on Tuesday and its only power plant had been put out of action.
Meanwhile, Palestinian factions Hamas, Fatah and Islamic Jihad are expected to meet in Cairo later to discuss a ceasefire.
