In the U.S., any three- to six-year secondary school serving students about 14–18 years of age. Four-year schools are by far the most common; their grade levels are designated freshman (9th grade), sophomore (10th), junior (11th), and senior (12th). Comprehensive high schools offer both general academic courses and specialized commercial, trade, and technical subjects. Most U.S. high schools are tuition-free, supported by state funds. Private high schools are usually classed as either parochial or preparatory schools.