Rami Abdel Rahman, head the Britain-based Observatory said, "Three missiles from a single air strike hit the school, where the underground level was being used as a shelter".
"Rescue workers are still searching for survivors".
There was no immediate comment from the government on the report by the Syrian Observatory for Human Rights, which could not provide casualty figures for the extremists.
In September 2015, Moscow began carrying out air strikes in support of Assad's ground troops and has since helped him recapture swathes of territory from rebels and jihadists.
The total number of civilians, mostly children, who have been evacuated from Eastern Ghouta since the start of the humanitarian operation has risen to 79,702, Russian defence ministry said on Tuesday.
Andrea Preti shock: "Claudia Gerini mi ha lasciato. Sono giorni che piango"
Insomma sembra proprio che Andrea Preti non riesca a trovare una persona che voglia stargli vicino senza abbandonarlo. Dopo un anno e mezzo ci siamo resi conto, con Claudia , che il nostro rapporto si era definitivamente consumato.
It said in a statement posted on its website that on Monday, 6,046 civilians left the district via humanitarian corridors.
/ AFP PHOTO / Hamza Al-AjwehHAMZA AL-AJWEH/AFP/Getty Images Member of the Syrian Civil Defense group carries a boy who was wounded during airstrikes and shelling by Syrian government forces, in Ghouta, a suburb of Damascus, Syria.
They have captured more than 80 per cent of the former opposition and have splintered the remaining territory into three sections, each held by a separate rebel group.
Syrian President Bashar Assad visited troops Sunday on the front line in the newly captured areas of eastern Ghouta, near the capital Damascus, according to images broadcast on state media and on the presidency sites.
The pocket where Arbin lies is held by the Faylaq al-Rahman extremist faction.