A United Nations inquiry has found that at least 44 Palestinians were killed by "Israeli actions" while sheltering at seven UN schools during last summer's war in Gaza.
UN Secretary General Ban Ki-moon said he deplored the deaths and stressed that UN facilities were "inviolable".
The inquiry also found that three empty UN schools were used by Palestinian militants to store weapons, and that in two cases they fired from them.
The 50-day conflict claimed the lives of more than 2,260 people.
At least 2,189 were Palestinians, including more than 1,486 civilians, according to the UN. On the Israeli side, 67 soldiers were killed along with six civilians.
In November, Mr announced that an independent board of inquiry would look into 10 incidents at schools run by the UN Relief and Works Agency (Unrwa), which works on behalf of Palestinian refugees, between 8 July and 26 August 2014.
Both Israel and Hamas, the militant group that dominates Gaza, said they would co-operate with the probe headed by the retired Dutch general Patrick Cammaert.
Although the board of inquiry's full 207-page report will remain private, the UN released a summary of its findings on Monday 27 April.
In one incident, a girls' school was hit by 88 mortar rounds fired by the Israel Defence Force, the summary said. Another girls' school was struck by an anti-tank projectile, while a third was hit by a missile.
At a fourth girls' school, the inquiry found, "no prior warning had been given by the government of Israel of the firing of [artillery] 155mm high explosive projectiles on, or in the surrounding area of the school".
"It is a matter of the utmost gravity that those who looked to them for protection and who sought and were granted shelter there had their hopes and trust denied," Ban wrote in a cover letter accompanying the summary.
Dismay at Palestinians' actions
He also expressed dismay that Palestinian militant groups would put UN schools at risk by using them to hide arms, calling this "unacceptable".
"United Nations premises are inviolable and should be places of safety, particularly in a situation of armed conflict," he warned. "I will work with all concerned and spare no effort to ensure that such incidents will never be repeated."
A spokesman for the Israeli foreign ministry, Emmanuel Nahshon, said in response: "All of the incidents attributed by the report to Israel have already been subject to thorough examinations, and criminal investigations have been launched where relevant.
"Israel makes every effort to avoid harm to sensitive sites, in the face of terrorist groups who are committed not only to targeting Israeli civilians but also to using Palestinian civilians and UN facilities as shields for their terrorist activities."