Punish any child who does this. Do not be afraid to actually issue a detention. Are you being stopped from actually issuing sanctions? The argument that because someone else is doing something wrong is wrong is fatuous.Tell them this in the corridor outside before they come in. Say "I am talking to you not them" Issue sanctions to any child who challenges the system. Make a point of persisting with any child tries to tell you what to do. Say "you are making it worse by arguing about it" Where is the support? Give every child who plays up a bad report. Have you spoken to the head of year/ SMT? Make them line up outside and take their reports off them before they come in ? Do you have seating plan ? I expect that you do but if not you need to make one. I know that there are only 4 corners in the room. Have you observed the class being taught by someone else? Do you have a starter task that they call all engage with? I imagine that you have a starter but a task that you can give then on a sheet and they can all start independantly of each other leaves them with no excuses. There is nothing wrong with asking a more experiences colleague to observe [without an official record] and give you advice. It is ok to ask "what is going wrong", what is wrong is when the teacher is blamed when the children choose to misbehave. Good luck.