Class, I want this done in four minutes, thirty seconds and counting...

...and then you get distracted by George, don't do that*, and it becomes an elastic four minutes, thirty seconds....unless you deploy a handy little online countdown tool like E.gg Timer.

Put in your own set time (three minutes and twenty seconds, five minutes, whatever)  on your own laptop/teacher PC and when time's up, it pings.  Whether or not you were distracted by George... and it's objective, so difficult to argue against.

Or you could get kids to open a new window on their machine and set individual timers.  Although that could have a cacophony effect...

Cheers

Ruth

* as referenced here.  Including a link to Joyce Grenfell's original monologue, lovely (and all-too-accurate) observational comedy.