High School Student, High Schooler
noun
You know 高校 means "high school." So, a "life" that does high school is a high school student. This is like how 中学生 are middle school students and 小学生 are elementary school students!
こうこうせい
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Use the reading of 高校 from before to learn the reading of this word as well, if it helps.
私の弟は高校生です。
My little brother is a high school student.
高校生の時は、テニス部に入ってました。
When I was in high school, I was in the tennis club.
今日は、高校生たちが、このパン工場の見学に来ます。パン工場の本気を見せましょう！
Today, high school students are coming to tour this bread factory. Let's show them what a bread factory can do!