Stanley and Diana join school working as substitute teachers (5,3)
I believe the answer is:
stand-ins
'stanley and diana join school working' is the definition.
I can't judge whether this definition defines the answer.
'substitute teachers' is the wordplay.
'substitute' becomes 'standin' ('stand-in' can be a synonym of 'substitute').
'teachers' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'standin'+'s'='STAND-INS'
'as' is the link.
Can you help me to learn more?
(Other definitions for stand-ins that I've seen before include "Locums" , "Substitutes for, say, actors" .)