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Is it possbile to call a student you taught before in your class 'my ex-student'?
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You could, and it would be understood, but I recommend "my former student."
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OK.

What about 'my old student', GG?
That's probably okay too, but it be prepared for things like "He's only 24! Hardly that old" as if the person didn't understand what you mean (although he did understand quite well).
Good. Thanks, GG!
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