ALEPPO, Syria — Inside the classrooms where they once studied, the boys darted like a pack. Their banging and clanking could be heard for a city block.

The playground outside had been hit by a Syrian Air Force airstrike, which fractured the school’s walls. Now the children were smashing the furniture, prying off wooden desktops and bench seats, rushing away with what they could.

The Isam al-Nadri School for Boys was being dismantled for the firewood it contained. One sixth grader, Ahmed, clutching the kindling he had made by ransacking a room, offered an irreducible argument for looting his own school. “I want heat,” he said.

Winter is descending on Aleppo, Syria’s largest city and the bloodied stage for an urban battle, now running into its sixth month, between rebels and the military of President Bashar al-Assad.