I'm assuming you follow the behaviour policy to the letter with no wiggle room - be ultra strict.
Teach in rows, not in groups. If tables are set out as groups, reorganise. They all need to be facing you, not each other. If you must have group work, they can move (but, if they are difficult, do no group work until you have a handle on them). You seat them, ideally boy then girl - do not let them choose their own seat.
Move quickly through tasks. Have something on the board that has to be copied into books as soon as they get in. Make it clear they will stay behind if it is not done/they need more time (for faff reasons, not SEN obviously).
Minimise your own movement - stay where you can see and set tasks they can do that you can oversee from the front or back (standing at the back, where they can't see you but you can watch over them can be very effective). I know this is not ideal, but it is the way to establish your authority and control of them.
Have everything prepared on PowerPoint/Notebook so you don't need to turn to write on the board.
Lavish praise - praise everything you can see that is good behaviour, on task, good work, good attitude. If there is no rewards system, get some stickers to give out.