The secondary level includes schools offering years 7 through 12 (year twelve is known as lower sixth) and year 13 (upper sixth). This category includes university-preparatory schools or "prep schools", boarding schools and day schools. Tuition at private secondary schools varies from school to school and depends on many factors, including the location of the school, the willingness of parents to pay, peer tuitions and the school's financial endowment. High tuition, schools claim, is used to pay higher salaries for the best teachers and also used to provide enriched learning environments, including a low student to teacher ratio, small class sizes and services, such as libraries, science laboratories and computers. Some private schools are boarding schools and many military academies are privately owned or operated as well.