one's high school days; high school years
Refers to the period of time when someone was a high school student. Often used in nostalgic or biographical contexts.
I met a friend from my high school days for the first time in a while.
He belonged to the baseball club during his high school years.
Refers to one's student days in general (elementary through university), while 高校時代 is specifically high school.
Refers to one's youth or adolescence more broadly, often with a poetic or nostalgic nuance, not limited to school life.
Compound of 高校 (high school) and 時代 (era, period).