The American school system
In the American school system there is primary or elementary school. This is followed by middle school or junior high. Then, at about age 14, students go to high school for 9th grade. The years in high school are named. The first year you are "freshmen", the second "sophomores", the third "juniors" and the fourth "seniors".
Miles is in the 11th grade, which is his "junior" year. At the end of high school, students receive the high school diploma. With this, students can go to college or university if they score high enough.
In the USA, there are public schools and private schools. However, only about eight to nine percent of students in the U.S. attend a private high school. This is probably due to the high school fees....