The fire, which officials said was likely caused by an electrical fault, raced through the building's wooden interior as panicked victims tried to jump from windows to safety.
Officials expressed concern that many of the dead were killed after they were unable to open a closed fire door to escape the top floors of the building.
Images showed scenes of devastation as emergency services arrived to tackle the fire at the dormitory building, parts of which were turned into a blazing wreck and whose roof collapsed.
"We lost 12 of our citizens in the fire. Eleven of them were schoolchildren and one was a tutor. 22 citizens are injured," Adana region governor Mahmut Demirtas said.
"According to initial findings, we believe the fire was caused by electrical fault," he said.
All 11 of the schoolchildren killed were girls. Their identities have yet to be disclosed but they were also said to be 14 or under.
The disaster took place in the town of Aladag north of Adana city, one of the biggest urban centres in the south of Turkey.