A few days ago, while attempting to resolve a dispute between two children, I reminded one that she had lied to cover her tracks on a previous occasion and had therefore lost a degree of credibility. Apparently, though I don't remember exactly what I said, I used the expression, 'You lied through your teeth' to this child (who is 9 by the way). To me, this is just an idiom: it has no more significance or strength than any other phrase. Unfortunately, however, I now find myself at the beginning of a disciplinary procedure because of it: the child's father has complained to the Head and wants to take the complaint to a 'higher' authority. What do you think? Is the phrase 'to lie through your teeth' so offensive as to justify disciplinary proceedings or am I the victim of an over-sensitive parent being too demanding?