Twenty-three bodies have been identified, Trairong Phiwpan, forensic science commissioner told reporters on Tuesday, with an investigation on the causes under way.
Sixteen students and three teachers were sent to a hospital for treatment, Transport Minister Suriya Juangroongruangkit said.
Images posted on social media and carried by local news outlets showed thick grey smoke pouring out of the bus, parts of which were still on fire.
The blaze was later extinguished.
A Reuters photographer saw fire trucks, police and rescue vehicles parked around the blackened vehicle, with a cluster of firefighters at the entrance.
"Teachers told us that the fire ignited very quickly," acting police chief Kittirat Phanphet told a press conference.
"From speaking to witnesses, we believe the explosion was caused by a spark from the tyre that lit the gas cylinder that was powering the vehicle," he said, adding that some students escaped through the window.
"We are investigating all individuals, including the bus company to see if this was a case of negligence."
The bus was a natural gas vehicle, according to Transport Minister Suriya.
Prime Minister Paetongtarn Shinawatra said the students were on a field trip from the province of Uthai Thani, about 250km north of the capital.
"As a mother, I would like to express my deepest condolences to the families," she said in a social media post on X.
