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But it wasn't a silly mistake was it? You deliberately lied and the HT realised and then you were honest enough to admit what you had done. As GW says you need to have support from your union if you are facing disciplinary procedures. Do not go into a hearing without your union rep and please meet with your union rep before hand to discuss how you will handle any meeting that is set up. With luck it will not mean that you have to leave the school but please be guided by your union.