On Russian state TV, images show a five-story building reduced to rubble. The sign on the damaged façade reads: "Starobilsk Professional College."
What happened early Friday has sparked Russian accusations, Ukrainian denials, an emergency UN Security Council session, and Kremlin vows of retribution.
Starobilsk is in Russian-occupied eastern Ukraine, in the Luhansk region Moscow claims to have annexed. Russian officials accuse Ukraine of a drone attack on the college dormitory, reporting 21 killed and 42 wounded.
Russian TV shows survivors in hospital, including 21-year-old Olga Kovaleva, rescued from under the rubble.
President Vladimir Putin called the incident a "terrorist strike," insisting there were "no military facilities" nearby.
Ukraine's General Staff stated it did carry out an attack near Starobilsk but maintains it struck a Russian military unit.
At Russia's request, an emergency UN Security Council session was held Friday. Russia's ambassador called the attack a "war crime," while Denmark's representative suggested Russia would need daily meetings on its own destruction across Ukraine.
Putin has ordered Russia's defense ministry to propose a response. Hawkish commentators urge retaliation beyond Ukraine, suggesting symbolic strikes against Europe.
Emergency teams completed search and rescue operations late Saturday.