The bus was carrying students from Uthai Thani province when a tyre burst on a highway in a northern Bangkok suburb, sending the vehicle crashing into a barrier, a rescue worker said in footage broadcast on local TV.
The bus was running on compressed gas and the crash ignited its fuel tanks, the rescue worker said.
Video footage from the scene showed flames engulfing the bus as it burned under an overpass, huge clouds of dense black smoke billowing into the sky.
“I have learned of the fire on a bus carrying students from Uthai Thani... resulting in deaths and injuries,” Paetongtarn wrote on X, formerly Twitter.
“As a mother, I would like to express my deepest condolences to the families of the injured and deceased.”
The blaze has been put out but rescue workers have had to wait for the bus to cool down before searching it for bodies, a rescue worker said.
Thailand has one of the worst road safety records in the world, with unsafe vehicles and poor driving contributing to the high annual death toll.