The fire broke out late on Sunday as the girls aged five to 12 slept at the Christian school in the northern province of Chiang Rai. The school is for girls from the area's hill-tribe villages.
"Most of the losses were because the children were asleep," said district police chief Colonel Prayad Singsin.
"Those that got out were awake," he said.
Two of the injured were in critical condition, he said.
The cause of the fire was not known, he said.