Superior school form (4-5)
I believe the answer is:
high-class
'superior' is the definition.
(I've seen this before)
'school form' is the subsidiary indication.
'school' becomes 'high' (high is a kind of school).
'form' becomes 'class' ('class' can be a synonym of 'form'**).
'high'+'class'='highclass'
(Other definitions for high-class that I've seen before include "Posh" , "Superior" .)