In the past 24 hours, at least 72 people have been killed in Aleppo, monitoring groups say.
Video released by Syrian Civil Defense Forces shows aftermath of alleged Russian air strikes.
The militant group is not restricted by the ongoing Syrian ceasefire.
Zeid Ra'ad Al Hussein says "deliberate starvation" is being used as a weapon of war.
A U.S.-Russia brokered cessation of hostilities has been in place since Saturday.
With Turkey and Saudi Arabia reluctant to send forces, the war may move well beyond Syria's borders.
Aid group says that up to 40 percent of the victims were women and children.
A new report shows that 11.5 percent of Syrians have been physically affected by the war.
Russian and Syrian government forces attack rebel-held areas around the Syrian city.
The school bombing came after heavy clashes between the Syrian government and opposition forces in Aleppo province.
Around 95 percent of Aleppo's doctors have been killed or detained, or have fled the city.