Thrashings concealed by popular heads of grammar schools (7)
I believe the answer is:
hidings
'thrashings' is the definition.
'concealed by popular heads of grammar schools' is the wordplay.
'concealed' becomes 'hid' ('hide' can be a synonym of 'conceal').
'by' is a charade indicator (letters next to each other).
'popular' becomes 'in' ('in' can mean trendy).
'heads of' indicates taking the first letters.
The initial letters of 'grammar schools' is 'gs'.
'hid'+'in'+'gs'='HIDINGS'
(Other definitions for hidings that I've seen before include "Beatings" , "concealed?" .)