What grades make up high school in the U.S. system?

Under the U.S. educational system, what grades constitute high school—typically encompassing which specific numerical grade levels (e.g., 9th through 12th), how does this align with the standard K-12 structure, and at what average ages do students generally attend these grades?

High school in the United States typically comprises four grades, which are as follows:

• Ninth grade (also called freshman year)
• Tenth grade (also called sophomore year)
• Eleventh grade (also called junior year)
• Twelfth grade (also called senior year)

These grades correspond to the traditional secondary education structure, generally attended by students aged 14–18. Upon completion, students receive a high school diploma, marking the transition to higher education or the workforce.