Had dinner in said school (5)
I believe the answer is:
eaten
'had dinner' is the definition.
(ate dinner)
'said school' is the wordplay.
'said' indicates a 'sounds like' (homophone) clue.
'school' becomes 'eton' (famous UK public school).
'eton' sounds like 'EATEN'.
'in' acts as a link.
(Other definitions for eaten that I've seen before include "used for food" , "demolished, possibly" , "Polished off" , "Crammed" , "having been fed?" .)