Standard of behaviour head teacher announced (9)

I believe the answer is:

principle

'standard of behaviour' is the definition.

'head teacher announced' is the wordplay.
'head teacher' becomes 'principal' (synonyms).
'announced' shows a homophone (sound like).
'principal' sounds like 'PRINCIPLE'.

(Other definitions for principle that I've seen before include "Set of rules" , "Basic standard" , "Morally correct behaviour" , "In which, theoretically" , "Axiom" .)

I've seen this clue in The Telegraph.

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