One tells where a schoolkid may be, with a set of books (9)
I believe the answer is:
informant
'one tells' is the definition.
(someone who tells on others)
'where a schoolkid may be with a set of books' is the wordplay.
'where a schoolkid may be' becomes 'in form' (form is a school class).
'with' is a charade indicator (letters next to each other).
'set of books' becomes 'NT' (New Testament).
'inform'+'a'+'nt'='INFORMANT'
(Other definitions for informant that I've seen before include "One who sings" , "Snitch" , "source of dope" , "Grass" , "Sneak" .)