Have a clock on the back classroom wall which is in view at all times. Tell the class how long they have for a task; for instance, "I'll stop you in 10 minutes". Then give time-checks throughout the exercise. "Five minutes gone, another five to go before I want some feedback."
This sends a clear message to the students that they will be asked to stop talking and that their discussion has to be focused on the task. Give them a final 60-second time check, then audibly count down the last five seconds.
You'll be lucky if the discussion stops, so you'll need a "silence" moniker. Each teacher has their own. Some clap their hands. Some shout "freeze" or "look at me". I stand with an arm in the air, naming students who are still talking. Thanking students who are already quiet also helps.
The toughest bit is to wait them out.
Make sure you have them focused for a reason. The teacher who gets a class quiet only to remind them to underline the title will find them less responsive next time silence is asked for.
Andy Day is head of humanities at Withernsea high school, East Yorkshire.
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