China's Shanghai has retained top spot in an international ranking of 15-year-old students, as the results of the latest Program for International Student Assessment (PISA) tests were published on Tuesday. Shanghai students came in first on reading, math and science, the three subjects covered in the triennial ranking by the Organization for Economic Co-operation and Development (OECD). It was followed by Singapore and China's Hong Kong and Taipei. Shanghai first topped the PISA tests in 2009. A total of 6,374 15-year-olds from 155 schools in Shanghai took part in the 2012 tests. These students did 13.8 hours of homework per week, the most among all the participants. Since 2000, the OECD has been evaluating the knowledge and skills of the world's 15-year-olds every three years. More than 510,000 students in 65 economies took part in the latest test in 2012. The PISA results will guide Shanghai's elementary educational reform, including a more balanced development of different levels of schools, reducing homework and giving more room for students' personal development, said Zhang Minxuan, president of the Shanghai Normal University and head of the PISA Shanghai program.