School’s alert inventor (4)

I believe the answer is:

bell

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This clue could be a double definition.

'school's alert' is the first definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'inventor' is the second definition.
(Alexander Graham Bell)

Can you help me to learn more?

(Other definitions for bell that I've seen before include "Metal ringer" , "''The ... Jar'' (Sylvia Plath)" , "eg Big Ben" , "Chime" , "Bird; man on board" .)

I've seen this clue in the Sydney Morning Herald.

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