He has been an alumni of the school where I taught.
He has been an alumni of the school where I used to teach.
Change 'has been' to 'is' (if he is still alive) or 'was' (if he is dead). Use either of the other two alternatives.
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Change 'has been' to 'is' (if he is still alive) or 'was' (if he is dead).
Use either of the other two alternatives.