School bell

A school bell is a signal used in schools to mark the beginning or the end of a school day or the end of a specific class.

In 2369, Miles O'Brien gave his wife Keiko O'Brien an antique school bell for her to use when she began running a on Deep Space 9. 

Keiko decided against using the bell due to the noise it would make on the Promenade early in the morning, keeping it for ornamental reasons. She would retain the bell as a reminder of her school career when the school closed and, when she began running the Andak Project on Cardassia, the bell adorned her desk in her office.