Shocks a quiet friend at school (6)
I believe the answer is:
appals
'shocks' is the definition.
('appal' can be a synonym of 'shock')
'a quiet friend at school' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'friend' becomes 'pal' (pal can mean a friend).
'at' means one lot of letters go next to another.
'school' becomes 'S'.
'a'+'p'+'pal'+'s'='APPALS'
(Other definitions for appals that I've seen before include "'Horrifies, outrages (6)'" , "Fills with disgust" , "Shocks" , "Fills with dismay or horror (6)" .)