Half the school get zero; unusual for music (7)
I believe the answer is:
scherzo
'for music' is the definition.
(scherzo is a kind of music)
'half the school get zero unusual' is the wordplay.
'half the' indicates taking half.
'get' means one lot of letters go next to another.
'unusual' indicates an anagram.
'school' halved is 'sch'.
'zero' anagrammed gives 'erzo'.
'sch'+'erzo'='SCHERZO'
(Other definitions for scherzo that I've seen before include "Lively passage" , "Light or playful movement" , "Playful musical movement" , "Vigorous and light movement (music)" , "Fast movement (music)" .)