Goodness in school's roast (6)
I believe the answer is:
scorch
'roast' is the definition.
(both can mean to get hot)
'goodness in school's' is the wordplay.
'goodness' becomes 'cor' (both are exclamations of surprise).
'in' indicates putting letters inside.
'school' becomes 'sch' (abbreviation).
'cor' going inside 'sch' is 'SCORCH'.
(Other definitions for scorch that I've seen before include "Burn on surface, char" , "Singe" , "Become burnt on the surface" , "go very fast" , "To make fast progress" .)