School punishment element observed in silence (6)
I believe the answer is:
gating
'school punishment' is the definition.
(confinement in a boarding school)
'element observed in silence' is the wordplay.
'element' becomes 'tin' (tin is a kind of element).
'observed in' is an insertion indicator.
'silence' becomes 'gag' (gagging is a kind of silencing).
'tin' inserted inside 'gag' is 'GATING'.
(Other definitions for gating that I've seen before include "Confining in school" , "punishment" , "Restriction of students" .)