school friend; classmate
Refers to a friend from school or university; slightly more formal than 同級生 or 友達, and often implies a lasting bond formed during school days.
He is a friend from my university days.
I met up with an old school friend for the first time in a while, and we had a great time reminiscing.
He was my father's school friend.
I reunited with a school friend for the first time in ten years.