The military said it had killed several "terrorists" in a "precision strike on a Hamas compound embedded inside a UNRWA school" in the Nuseirat area while Gaza's Al-Aqsa Martyrs hospital said at least 37 people were killed in the strike.
"Another UNRWA school turned shelter attacked," Lazzarini wrote on social media platform X. "Attacking, targeting or using UN buildings for military purposes are a blatant disregard of International Humanitarian law."
He added that UNRWA "shares the coordinates of all its facilities (including this school) with the Israeli army and other parties in the conflict".
"Targeting @UN premises or using them for military purposes cannot become the new norm," said Lazzarini, who added that the school was sheltering 6,000 displaced people when it was hit.
Israeli rights group B'Tselem called the strike a "suspected war crime".
"If, as Israel claims, Hamas used the school to plan military operations, this action is illegal, but it cannot justify the massive harm to civilians who sought shelter in the school from the horror of prolonged fighting," the group said in a statement.
"As demonstrated throughout the war, the killing of civilians is an unavoidable result of the character of Israel's military activity in the Gaza Strip," it said, urging the international community to help stop the fighting.