Rather formal attacks upset members of school (8)
I believe the answer is:
stiffish
'rather formal' is the definition.
I can't judge whether this definition defines the answer.
'attacks upset members of school' is the wordplay.
'attacks' becomes 'fits' (fit is a kind of attack).
'upset' shows that the letters should be reversed in order.
'members of school' becomes 'fish' (I can't justify this - if you can you should give a lot more credence to this answer).
'fits' in reverse letter order is 'stif'.
'stif'+'fish'='STIFFISH'
Can you help me to learn more?
(Another definition for stiffish that I've seen is " quite starchy".)