It’s unfortunate about Grammar School being a complete mess (6)
I believe the answer is:
pigsty
'a complete mess' is the definition.
'pigsty' can be an answer for 'mess' (I have seen 'Proverbial mess' mean 'pigsty' so perhaps 'mess' could also mean 'pigsty'). I'm not certain of the 'a complete' bit.
'it's unfortunate about grammar school' is the wordplay.
'it's unfortunate' becomes 'pity' (I can't explain this - if you can you should believe this answer much more).
'about' indicates putting letters inside.
'grammar school' becomes 'gs' (abbreviation for grammar school).
'pity' placed around 'gs' is 'PIGSTY'.
'being' is the link.
Can you help me to learn more?
(Other definitions for pigsty that I've seen before include "dump" , "Very dirty abode" , "Proverbial mess" , "Untidy place" , "Dirty domicile" .)