Go off school during appointment (8)
I believe the answer is:
detonate
'go off' is the definition.
(I know that go off can be written as detonate)
'school during appointment' is the wordplay.
'school' becomes 'eton' (famous UK public school).
'during' indicates putting letters inside.
'appointment' becomes 'date' (synonyms).
'eton' going inside 'date' is 'DETONATE'.
(Other definitions for detonate that I've seen before include "go off with a bang" , "Trigger" , "let off" , "Blow up, explode" , "Cause to explode" .)