Fair then bad in school exam (2,3,5)
I believe the answer is:
on the level
'fair' is the definition.
(I've seen this before)
'then bad in school exam' is the wordplay.
'bad' indicates anagramming the letters.
'in' indicates putting letters inside.
'school exam' becomes 'olevel' (I've seen this before).
'then' with letters rearranged gives 'nthe'.
'nthe' inserted within 'olevel' is 'ON THE LEVEL'.
(Other definitions for on the level that I've seen before include "Some horse racing is" , "Honest, truthful (colloq.)" , "Honest - genuine" , "Fair; honestly speaking" , "No tipping" .)