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.
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.