A hundred girls in school (7)

I believe the answer is:

classes

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'a hundred girls in school' is the definition.
I can't judge whether this defines the answer.

'a hundred girls' is the wordplay.
'a hundred' becomes 'C' (Roman numeral).
'girls' becomes 'lasses' (lass can mean a girl).
'c'+'lasses'='CLASSES'

Can you help me to learn more?

(Other definitions for classes that I've seen before include "Categories" , "Groups with the same social or economic status" , "See 1" .)

I've seen this clue in The Globe and Mail.

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