Intelligence in school? That's a change (6)
I believe the answer is:
switch
'a change' is the definition.
(switch is a kind of change)
'intelligence in school?' is the wordplay.
'intelligence' becomes 'wit' (wit is a kind of intelligence).
'in' indicates putting letters inside.
'school?' becomes 'sch'.
'wit' going within 'sch' is 'SWITCH'.
'that's' is the link.
(Other definitions for switch that I've seen before include "Device for changing electrical connections" , "birch" , "Change; hair-piece" , "Change - debit card" , "Device for opening and closing electric circuits" .)