In Shanghai, the nine years of compulsory education—including five years of primary education and four years of junior secondary education—are free, with a gross enrollment ratio of over 99.9%. The city's compulsory education system is among the best in the world: in 2009 and 2012, 15-year-old students from Shanghai ranked first in every subject (math, reading, and science) in the Program for International Student Assessment, a worldwide study of academic performance conducted by the OECD. The consecutive three-year senior secondary education is priced and uses the Senior High School Entrance Examination ("Zhongkao") as a selection process, with a gross enrollment ratio of 98%. Among all senior high schools, the four with the best teaching quality—Shanghai High School, No. 2 High School Attached to East China Normal University, High School Affiliated to Fudan University, and High School Affiliated to Shanghai Jiao Tong University—are termed "The Four Schools" () of Shanghai. , the city's National College Entrance Examination ("Gaokao") is structured under the "3+3" system, in which all general senior high school students study three compulsory subjects (Chinese, English, and math) and three subjects chosen from six options (physics, chemistry, biology, history, geography, and politics).