Far fewer sirens signaling incoming rockets sounded in Israel on Wednesday than in previous days. But in Gaza, there was little sign of a letup. Among the casualties were 10 members of the Astal family who were killed by an Israeli missile as they huddled in their diwan, or meeting room, according to news reports and the Health Ministry.

Jabaliya, a refugee camp just north of Gaza City, has been under intense shelling since Tuesday afternoon, with 50 people killed over a 24-hour period, health officials said. Already one of Gaza’s most densely crowded areas, its streets were packed in recent days with people who had fled their homes closer to the border. More and more had crowded into the Abu Hussein girls’ elementary school.

Mr. Turner of the United Nations said his agency had provided the GPS coordinates of the school to the Israel Defense Forces 17 times, first on July 16 and most recently on Tuesday at 8:48 p.m. Ziad Yousef, who also works for the agency, said the doors had been locked at 11 p.m. on Tuesday so no one could come or go.

At least four strikes hit in close succession in a straight line across the school compound, indicating artillery fire, according to people who saw the attack. The drop ceiling of one classroom had collapsed, and the tin roof was peppered with shrapnel holes. The ground was covered with rubble, clothing and pools of blood.

At the nearby Kamal Adwan hospital, Saeed Adham stood over the bed of his 15-year-old son, Rizek, whose right leg had been shattered by shrapnel. An X-ray of Rizek’s calf showed bones looking like an archipelago. Mr. Adham said his wife and other children remained at Abu Hussein despite the danger. “We have nowhere but the school,” he said.

Mr. Turner said he hoped that United Nations experts, criticized in the two previous instances by Israel and its supporters for having given rockets found in two schools to Gaza-based security officials, would be able to dispose of the rockets found in the third school.