I'm stunned that anyone would even post such a query! If I was your HT and saw this post you would now have ruined any chance of a permanent post. OF COURSE you are not to tell your class that all rules are suspended and they can do as they please, it is up to the visitor to set rules! Do you say the same to them when they have a supply teacher? "Hey it isn't me so my rules don't apply. Do as you please and it is up to the visiting teacher to deal with it. Nothing to do with me." Your HT is (or should be) well aware of the behaviour of all classes in the school and the reasons for the behaviour. They will also have references for the candidates, who may well be better than you at behaviour management. It is their job to choose the best person for the post based on everything they know. It is for them to decide about which class the new teacher will have and who would be best suited to it. Your job is simply to write the best application you can, teach the best lesson you can, tell your class they will have a visitor teaching them and will have a lovely time and that some other people might be watching them, prepare well and perform brilliantly in the interview. That is it. Nothing more.