Stiff punishment at school? Bad luck (4,5)
I believe the answer is:
hard lines
'bad luck' is the definition.
(phrase used to express commiserations or sympathy)
'stiff punishment at school?' is the wordplay.
'stiff' becomes 'hard' (similar in meaning).
'punishment at school?' becomes 'lines'.
'hard'+'lines'='HARD LINES'
(Other definitions for hard lines that I've seen before include "Bad luck" , "Script full of tongue-twisters?" , "Tough" .)