Sound is produced when your school bell is struck with a hammer. Why?

Hint: Vibration is thus produced when mechanical energy is given to some object that allows it to vibrate and produce disturbance in the particles to produce sound.

Complete solution
The vibration of particles produces sound. Sound waves travel in medium. It does not travel in vacuum. So, it travels through the air. When the bell is struck with the hammer the molecules of the bell start vibrating. These vibrations are produced when the bell disturbs the air around it. The vibrating particles put force on each other and in this way disturbed air molecules, move forward and backward. This back and forth motion of molecules results in production of sound and reaches to the ears.

Note: The phenomenon of refraction is there in the particles of air and also there is compression of the air particles.