In high school there are four grades. The first grade is nine and is also called freshman year. After ninth grade is 10th and is called sophomore year. Junior year is the name for 11th grade and then the last grade of high school is 12th and is called senior year.
Once the four years of high school are completed then formal education is over for the student in the United States.
Any education after high school is optional.
What Is Before High School?
Before high school is middle school/junior high and are grades seven, eight, and sometimes six.
I explain the middle school grades in more detail in this article and also this one.