Work out zero in school (5)
I believe the answer is:
train
I believe this clue is a triple definition!
'work out' is the first definition.
(training is a kind of working out)
'zero in' is the second definition.
(both can mean to focus on with a camera)
'school' is the third definition.
(training is a kind of schooling)
(Other definitions for train that I've seen before include "Aim < to prepare < railway carriages" , "See 17" , "Locomotive and coaches" , "Teach (person) a skill" , "Coach - coaches with engine" .)