'I am present', as the schoolboy said (5)

I believe the answer is:

adsum

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'i am present as the schoolboy said' is the definition.
I can't judge whether this defines the answer.

'present as the schoolboy said' is the wordplay.
I cannot really see how this works, but
'present' could be 'ad' (AD, Anno Domini, as opposed to BC, Before Christ) and 'ad' is present in the answer.
The remaining letters 'sum' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for adsum that I've seen is " here".)

I've seen this clue in the Evening Standard.

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