An air raid on a rebel-held neighbourhood of Syria's northern city of Aleppo hit a school, killing at least 18 people, 10 of them children, Wednesday, a monitoring group said. At least one teacher was also among the dead in the raid on the Ansari district of the former commercial hub, the Syrian Observatory for Human Rights said. Rebel-held areas of the city have come under repeated attack from the air since mid-December. The government's use of barrel bombs, unguided munitions generally dropped from helicopters flying at altitude to avoid ground fire, has come in for particular criticism from human rights watchdogs. Because they cannot be directed against military targets, they exact an indiscriminate toll on civilians, which Human Rights Watch has said makes them unlawful.