I have recently started my first teaching post in a fairly challenging secondary school and I am really struggling to get the class to be quiet when I need to talk. I refuse to start explaining something until everyone is quiet and paying attention, but it takes so long that I have wasted so much lesson time. I do implement sanctions according to the school behaviour policy and I do praise the ones who are following instructions, but I am always waiting for a few who just can't shut up. It's unfair on those who do want to learn. Please help.