It is the second deadly Israeli airstrike to hit a UN school in the past few hours.
Palestinian health official Said Joudeh says three people were killed and four wounded in the airstrike in the northern Gaza town of Jebaliya.
He says the school was turned into a shelter for people displaced by an Israeli offensive against the Hamas militant group.
More than 500 people have been killed, including dozens of civilians.
Israel isn't commenting on the latest airstrikes. But it has accused Hamas of using schools, mosques and residential areas for cover.
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