The gong itself rings at a certain fundamental frequency, say 1 or 2 kHz. It has a clanging quality which is due to the presence of non-harmonic overtones (meaning they are not exact multiples of the fundamental frequency).
It is struck by the clapper many times per second, say 10 or 20 Hz.
I think you'll need to combine several sine waves, then turn the lot on and off very rapidly, to imitate the school bell.
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As I think about it, the sound should not entirely turn off between strikes of the clapper. Simply make the clapper cause a spike (instant jump) in amplitude of the waveform.