Set off from school patrolled by fuddy-duddy (9)

I believe the answer is:
detonated
'set off' is the definition.
(I've seen this before)
'school patrolled by fuddy-duddy' is the wordplay.
I cannot quite see how this works, but
'school' could be 'eton' (famous UK public school) and 'eton' is present in the answer.
'by' could be 'at' and 'at' is found within the answer.
This explanation may well be incorrect...
'from' acts as a link.
Can you help me to learn more?
(Other definitions for detonated that I've seen before include "Set off" , "let off" , "Blown up" .)
