This is the 67th most frequent Korean word.
동안
"동안" means "during" or "for a period of time."
Here, '동안' is used to indicate the time span of 'school years' during which the action took place.
저는 학창 시절 동안 열심히 공부했어요.
I studied hard during my school years.
In this example, '동안' specifies the duration of 'lunchtime' when the activity happens.
그는 점심 시간 동안 운동을 합니다.
He exercises during lunchtime.
The word '동안' here is used to describe the duration 'while spending time together' during which something is observed.