Thai students scored 441 in reading, 427 in mathematics, and 444 in science in the Pisa report officially released in Thailand on Wednesday.
It is an improvement from the previous test in 2009, when Thais scored 421 in reading, 419 in maths and 425 in science.
But it is still below the average scores of countries in the Organisation for Economic Co-operation and Development (OECD). The average scores there are 496 in reading, 494 in maths and 501 in science. Girls performed significantly better than boys in all three subjects.
Held by the OECD, Pisa is conducted every three years in 65 countries and economies. Thailand ranked 48th in reading, 50th in maths and 47th in science in the results of the 2012 tests release on Wednesday.
This time it assessed the competencies of 6,606 Thai students aged 15 in the three subjects to examine.
China's Shanghai had the highest scores in reading with 570 points, maths with 613 points and science with 580 points.
Asian countries ranked in the top 10 in the three categories - China's Shanghai, Singapore, Hong Kong, Taiwan, South Korea and Japan.
Thailand has been a participant in the Pisa test since 2000.
Sunee Klainin, head of National Project Manager Pisa 2000-2012, on Wednesday pointed to better teaching at schools supervised by the Office of the Basic Education Commission and greater opportunities for learning as factors contributing to the improvement in the scores.
She noted that the scores also reflected a gap between schools in Bangkok and other towns, and in other countries.
"It echoes a disparity in distribution of quality education between Bangkok and big towns, and elsewhere.
"What is interesting is those in schools with education expansion opportunies show a significant improvement as they gained far higher scores, resulting in the country's higher Pisa score," she said.
Education Minister Chaturon Chaisang said that the Pisa scores will be analysed to as part of efforts to improve the education system.
The minister was upbeat, saying the Thai ranking demonstrated the country's potential and could attract investment.
The minister said he was impressed by Vietnam's education level. It had entered the Pisa test for the first time and scored very well in reading with 508 points, maths with 511 points and science with 528 points.