I would say "stand-in teacher", not "sit-in teacher". I don't know the term "teach-in" in the sense you are suggesting. org/wiki/Teach-in
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Kinston AG'sit-in teacher'
Kinston AG'fill-in teacher'These seem to be regional terms. They are not known in all parts of the English-speaking world.
Kinston AGIs 'sit-in teacher' acceptable in English?No.
Kinston AGif a regular subject teacher is absent or away, another teacher from the same school is assigned to cover for the absent teacher during his free time.The term is "substitute teacher" in the U.S.
Kinston AGteach-inThis is