These words are used to describe what year of school someone is in. These terms apply to the four years of a standard high school.
9th grade is freshman year
10th grade sophomore year
11th grade junior year
12th grade senior year
In the United States, there are 12 years of mandatory schooling. The first eight are referred to by numbers (1st grade, 5th grade, 8th grade). After entering high school, grades 9 through 12 have alternate names for students - freshman, sophomore, junior and senior.