Mess made by quiet group in school (7)
I believe the answer is:
splotch
'mess' is the definition.
(a splotch is a stain or messy spot)
'quiet group in school' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'group' becomes 'lot'.
'in' indicates putting letters inside.
'school' becomes 'sch'.
'p'+'lot'='plot'
'plot' inserted within 'sch' is 'SPLOTCH'.
'made by' is the link.
(Other definitions for splotch that I've seen before include "Mark" , "Spot, splash, smear" .)