Really bad goal, one let in by members of school (8)
I believe the answer is:
fiendish
'by members of school' is the definition.
I can't judge whether this definition defines the answer.
'really bad goal one let in' is the wordplay.
'really bad' becomes 'fish' (I can't justify this - if you can you should believe this answer much more).
'goal' becomes 'end' (both can mean a purpose).
'one' becomes 'i' (Roman numeral).
'let in' indicates putting letters inside.
'end'+'i'='endi'
'fish' placed around 'endi' is 'FIENDISH'.
Can you help me to learn more?
(Other definitions for fiendish that I've seen before include "Terrible" , "like some Sudokus" , "Diabolic" , "Finished (anag) - demonic" , "Like Satan or the devil" .)