The pre-dawn accident occurred when the bus lost control after its brake system malfunctioned while it was cruising down a mountain road and crashed into the rear of the 18-wheel truck.
The bus was taking schoolgirls, all under the age of 10, and their teachers from Ban Dong Lob school on a sight-seeing trip to Chomthien Beach, Pattaya.
Eleven people were killed on the spot and four others died at a local hospital, reports said.
A police officer said initial investigation suggested the crash was likely caused by faulty brakes.
Thailand has the second highest fatality rate from road crashes in the world, after Namibia, according to a report published this month by the Transportation Research Institute of the University of Michigan.