Go off school in time (8)
I believe the answer is:
detonate
'go off' is the definition.
(thesaurus)
'school in time' is the wordplay.
'school' becomes 'eton' (famous UK public school).
'in' indicates putting letters inside.
'time' becomes 'date' (date is a kind of time**).
'eton' placed inside 'date' is 'DETONATE'.
(Other definitions for detonate that I've seen before include "Trigger" , "Set off explosive device" , "Start 15 [EXPLOSION]" , "go off with a bang" , "Discharge" .)