Judge fools at school (6)
I believe the answer is:
assess
'judge' is the definition.
(to assess is to judge or rate)
'fools at school' is the wordplay.
'fools' becomes 'asses' (ass can mean a foolish person).
'at' says to put letters next to each other.
'school' becomes 'S'.
'asses'+'s'='ASSESS'
(Other definitions for assess that I've seen before include "Remedy for all ills" , "Place a value on" , "Test" , "Reckon value" , "(Of a tax) determine the amount to be paid" .)