Getting on coach is traditional (3-6)
I believe the answer is:
'traditional' is the definition.
(I've seen this before)
'getting on coach' is the wordplay.
'getting on' becomes 'old' ('getting on a bit' means 'old').
'coach' becomes 'school' (both can mean to teach or train).
'is' acts as a link.
(Another definition for old-school that I've seen is " Such as attitudes from earlier days".)