At least 18 people, including 10 children, have been killed in a Syrian government air strike in the northern city of Aleppo, activist groups say.
A missile struck Ain Jalout school in the Ansari district, the Syrian Observatory for Human Rights reported.
Images showed blood on corridor walls and debris in classrooms.
Meanwhile, the UN's humanitarian chief has said the Security Council resolution passed two months ago to boost aid to Syria is not working.
"Far from getting better, the situation is getting worse," Valerie Amos warned, noting that less than 10% of the 240,000 people living in besieged areas had received much-needed aid in the past four weeks.
She said the resolution, which demanded more cross-border access for aid convoys, was being violated by all parties and called on the Security Council to take further action.
The BBC's Nick Bryant in New York says Russia, as a permanent Security Council member, would almost certainly block a tougher resolution including punitive measures such as sanctions.
Some Western diplomats would like the UN to do more to implement the existing resolution by sending aid convoys across the border into Syria, even without the permission of the Syrian government, our correspondent adds.
Wednesday's air strike in Aleppo came as both sides try to break the stalemate that has existed since fierce fighting first erupted in the city in July 2012.
The Syrian Observatory, a UK-based group, put the death toll at 18 and said most of the children who died were aged between 12 and 13. A local opposition activist group, the Aleppo Media Center, said 25 people had been killed, most of them children.
Videos of the aftermath released by opposition activists showed walls and floors splattered in blood, twisted metal and scattered artworks.
The school had been hosting an exhibition of drawings and paintings by pupils at schools around the area.
Another video showed the bodies of 10 children wrapped in sheets on the floor of a hospital ward.
On Tuesday, 14 children were killed and more than 80 injured when mortar shells hit the Badr al-Din al-Hussein technical institute in the Damascus suburb of Shaghour, state media said.
Another mortar attack in the Adra area, on the outskirts of the capital, killed three children at a shelter for internally displaced families.
Later, a car bomb exploded in a busy part of the central city of Homs, leaving at least 100 people dead, including many women and children.
The United Nations Children's Fund (Unicef) has expressed outrage.
"Every day, across Syria, children who are simply trying to go about their everyday lives are being killed and maimed by indiscriminate attacks on populated areas," said Maria Calivis, Unicef's regional director for the Middle East and North Africa.
"These attacks appear to be escalating, in complete disregard of all the calls that have been made to stop this insane cycle of violence, and to avoid similar breaches of international law."