こうこうせい高校生koukousei
1.
A student who is enrolled in the upper secondary school system, typically between the ages of 15 and 18, and is preparing for entrance into higher education or the workforce.
2.
An individual attending a school that focuses on educational subjects required for graduation while also engaging in extracurricular activities, shaping both their academic and social skills during the late teenage years.
3.
A young person who has completed their compulsory education and is pursuing advanced studies in a school aimed at providing comprehensive education before university or vocational training.
Example:
彼らの大部分が高校生だった。