Stood up and was counted in school for stab at what's common knowledge (5,3)

'what's common knowledge' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'stood up and was counted in school for stab' is the wordplay.
I cannot quite see how this works, but
'school' could be 's' and 's' is present in the answer.
'stab' could be 'poke' ('poke' can be a synonym of 'stab') and 'poke' is found within the answer.
The remaining letters 'out' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).

'at' is the link.

Can you help me to learn more?