High school students are required to take a wide variety of courses in English, mathematics, science, and social science. They may also be required to take foreign language or physical education, and they may elect to take music or art courses. Many high schools also offer vocational training courses. A course can be one semester or two semesters in length. The academic year generally begins in late August and ends in early June. In the United States, education is compulsory for all students until age 16. According to the US Census Bureau, 85% of students graduate from high school. Most high school students graduate at the age of 17 or 18 years old. A student graduates after he or she has successfully passed all of the required courses. Grades are given to students for each course at the end of each semester. The grading scale is A-excellent, B-above average, C-average, D-below average, and F-failing. A student who fails a required course must take the course again.