Sensible grade by teacher of French is put up (5-6)
I believe the answer is:
level-headed
'sensible' is the definition.
(thesaurus)
'grade by teacher of french is put up' is the wordplay.
'grade' becomes 'level' ('level' can be a synonym of 'grade'**).
'by' says to put letters next to each other.
'teacher' becomes 'head'.
'of french' becomes 'de'.
'is put up' says the letters should be written in reverse.
'de' reversed gives 'ed'.
'level'+'head'+'ed'='LEVEL-HEADED'
(Other definitions for level-headed that I've seen before include "Calm and sensible" , "Showing good judgement" .)