In United States education, a senior is a student in the fourth and final year of study (generally high school or college/university study).
Senior (education) Wikipedia
In the United States, the twelfth grade is usually the fourth and final year of a student's high school period and is referred to as his/her senior year. In England and Wales, students in their tenth year and above in Secondary School are seniors; in Scotland, students in their fifth year and above are seniors. In the Canadian province of Ontario, high school students in their third year and above are considered to be seniors. Many schools grant special privileges to seniors, such as reserved parking spaces and lunch tables.
In the United States, the final year of a student's education towards a bachelor's degree, typically the fourth year, is known as the senior year.
In college athletics, a student in their final year of eligibility is known as a senior.
The term super senior is used in the United States to refer to a fifth-year student who has not completed the graduation requirements by the end of the fourth year, and thus is required to stay an additional year to complete said requirements.