The latest possible moment when secondary school starts (4,4)
I believe the answer is:
high time
'the latest possible moment' is the definition.
'high time' can be an answer for 'moment' (high time is a kind of moment). I'm unsure of the 'the latest possible' bit.
'when secondary school starts' is the wordplay.
'when' becomes 'time' (I am not sure about this - if you are sure you should believe this answer much more).
'secondary school' becomes 'high' (high is a kind of secondary school).
'starts' is a charade indicator (letters next to each other).
'time' put after 'high' is 'HIGH TIME'.
Can you help me to learn more?
(Other definitions for high time that I've seen before include "'The latest possible moment (4,4)'" , "Beyond the proper hour, but not too late - check lofty clock" , "long overdue" .)