Make progress at a good school (3,2)
I believe the answer is:
get on
'make progress' is the definition.
(synonyms)
'at a good school' is the wordplay.
'at' means one lot of letters go next to another.
'a good' becomes 'g' (abbreviation).
'school' becomes 'eton' (famous UK public school).
'g' put next to 'eton' is 'GET ON'.
(Other definitions for get on that I've seen before include "Board vehicle or make progress" , "Board; establish good relations" , "Make progress or mount up" , "Make progress or climb aboard" , "Agree - embark - make progress" .)