timer school bell

Are you referring to an alarm clock that needs to be set for your time to get up for school? If so, if it is a wind up clock there should be a stem on the back that you turn to set the alarm time and another stem that you pull out to arm the alarm clock and push in to turn the alarm off when it is ringing.

If it is an electric/electronic clock, look for labels on the various buttons for setting the alarm time and for arming the alarm clock.

If you're referring to something else, we need a bit more detail.

Are you talking about the large clock on a classroom wall?

Or, if it's the one that automatically sounds a bell at beginning and end of classes, it used to be controlled by a master clock with moveable brass keys that tripped a lever to sound the bell.

In other words, we could use a little more detail.