The United Nations Relief and Works Agency told CNN that most of the casualties were outside the school in the Jabalya refugee camp. The camp has been a target in the ongoing Israeli military campaign against Palestinian militants in Gaza.
UNRWA director John Ging said in a briefing the school was a U.N. facility, adding, "It's a very built-up area, so of course it was entirely inevitable that if artillery shells landed in that area, that there would be a high number of casualties."
Ging called for an independent investigation into the strike, while Israeli officials told the U.S. broadcaster they were looking into the incident, which came as Israeli military ground forces moved to surround Gaza City and warplanes continued their heavy bombing campaign overnight.
Gaza City, with a population of roughly 500,000 people, was being encircled by Israeli troops after at least 50 strikes from the Israeli Air Force were directed at Gaza positions held by the Palestinian militant group Hamas, witnesses said.
The Israeli military said it sustained casualties from so-called "friendly fire." Three Israeli soldiers died when a tank shell hit a building they were in, The New York Times reported.
Hamas, which controls the Gaza Strip, has killed five Israelis since the conflict began Dec. 27, the Times said. Militants claimed they fired four rockets into Israel overnight.
Since its ground assault began Saturday, Israel said it has killed 130 Hamas fighters. Israeli officials also said planes destroyed smuggler tunnels in the south.
European diplomats traveled to the region seeking a cease-fire while both Israeli and Hamas leaders spoke of victory, the Times said. Recorded messages from the Israeli military told Gazans "We are getting rid of Hamas."
Hamas leader Mahmoud Zahar, speaking in hiding, said in a televised recorded speech, "The Israeli enemy in its aggression has written its next chapter in the world, which will have no place for them."
Palestinian medical officials estimated the death toll since the campaign began in December topped 550 as of Monday. The United Nations estimated about a quarter of the dead were civilians.
The Red Cross said water supplies for 500,000 Gaza residents are expected to run out soon, The Washington Post reported. Conditions inside the territory are difficult to confirm because Israel has banned foreign journalists.