People watch as smoke billows following an Israeli strike in Jabalia, in the northern Gaza Strip. AFP
The strike on the school in the Daraj neighbourhood of Gaza City also wounded more than 55 people, said Fahmy Awad, head of the ministry’s emergency service.
He said a father and his five children were among the dead.
He said the school was hit three times while people slept, setting their belongings ablaze.
Footage circulating online showed rescuers struggling to extinguish fires and recovering charred remains.
A separate strike on a home killed 15 members of the same family, including five women and two children, according to Shifa Hospital in Gaza City, which received the bodies.
Palestinian resistance fighters, meanwhile, fired three projectiles from Gaza, two of which fell short within the territory and a third that was intercepted, according to the Israeli military.
Israel renewed its war in March after unilaterally ending a ceasefire with Hamas.
It began allowing a trickle of humanitarian aid into Gaza last week after blocking all food, medicine, fuel or other goods from entering for two and a half months. Aid groups have warned of famine and say the aid that has come in is nowhere near enough to meet mounting needs.
A new aid system supported by Israel and the United States but rejected by U.N. agencies and aid groups is expected to begin operations as soon as Monday, despite the resignation of the American leading the effort, who said it would not be able to operate independently.
Israel says it plans to seize full control of Gaza and facilitate what it describes as the voluntary migration of its over 2 million population, a plan rejected by Palestinians and the Arab and international community.
Israel's war has destroyed vast areas of Gaza and internally displaced some 90% of its population. Many have fled multiple times.
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