Obviously if a person is your classmate now then you cannot call them a "former classmate", so I suppose you are asking about someone who was a classmate in the past -- whether you can use "classmate" and "former classmate" interchangeably in that case. If you call someone a "classmate" with no further context, then it sounds as if you mean current classmate, so you would need to include the word "former". g.
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