The health ministry said the attack on Saturday at the school in Al-Nuseirat wounded more than 50.
The Israeli military said it targeted gunmen operating in the vicinity, after taking precautions to minimise risk to civilians.
Mahmoud Basal, spokesman of the Gaza Civil Emergency Service, said in a statement that the number of dead could rise because many of the wounded were in critical condition.
The attack meant no place in the enclave was safe for families who leave their houses to seek shelters, he said.
Al-Nuseirat, one of Gaza Strip's eight historic refugee camps, was the site of stepped-up Israeli bombardment on Saturday.
An air strike earlier on a house in the camp killed at least 10 people and wounded many others, according to medics.
Children's mental health in — Save the Children International (@save_children) #Gaza is being pushed beyond breaking point after nearly 9 months of war.Bisan shares how children are trying to cope without access to school or safe places to play.We won't stop calling for a #CeasefireNOW, it's the only way to save lives. pic.twitter.com/5QC3ooDnHsJuly 6, 2024
In its daily update of people killed in the nearly nine-month-old war, the Gaza health ministry said Israeli military strikes across the enclave killed at least 29 Palestinians in the past 24 hours and wounded 100 others.
Among those killed in separate air strikes on Saturday were five local journalists, raising the toll of journalists killed since October 7 to 158, according to the Hamas-led Gaza government media office.
Gaza health authorities say more than 38,000 Palestinians have been killed in Israel's offensive.
The health ministry does not distinguish between combatants and non-combatants but officials say most of the dead are civilians.
Israel has lost 323 soldiers in Gaza and says at least a third of the Palestinian dead are fighters.
Israel launched its offensive, aimed at eliminating the militant Islamist group Hamas, in response to a Hamas-led assault on Israel on October 7 in which 1200 people were killed and over 250 taken hostage, according to Israeli tallies.
Israeli forces, which have deepened their incursions into Rafah, in the south of the enclave near the border with Egypt, killed four Palestinian policemen and wounded eight others, in an air strike on their vehicle on Saturday, health officials said.
A statement issued by the Hamas-run interior ministry said the four included Fares Abdel-Al, the head of the police force in western Rafah neighbourhood of Tel Al-Sultan.
The Israeli military said forces continued "intelligence-base operations" in Rafah, destroyed several underground structures, seized weapons and equipment, and killed several Palestinian gunmen.
Israel has said its operations in Rafah aim to eradicate the last Hamas armed wing battalions.
The Israeli military said it eliminated a Hamas rocket cell in Deir Al-Balah in central Gaza that operated from inside a humanitarian-designated area.
It said it carried out a precise strike after taking measures to ensure civilians were unharmed.
Hamas denies Israeli accusations that it uses civilian properties and facilities for military purposes.
The armed wings of Hamas and the Islamic Jihad said fighters attacked Israeli forces in several areas of Gaza with anti-tank rockets and mortar bombs.
