This is the 5749th most frequent Korean word.
벨
"Bell" or "buzzer."
Here, '벨' refers to a doorbell or a device used to signal someone.
그녀는 벨을 눌러 문을 열었다.
She pressed the bell to open the door.
In this context, '벨' is used to denote a school bell that signals the start or end of a class.
교실의 숙제를 한 다음에 벨이 울릴 때까지 기다리세요.
After finishing the class homework, wait until the bell rings.
It represents a standard ringing bell, possibly used here as a prank device.