Or the first 20 minutes you would routinely do with any new class.Don't throw away time on your classroom 'rules' or an ice-breaker.
One issue that can floor candidates - and it really shouldn't - is finding out that the group have already studied the topic, or (even worse) just taken part in another sample lesson on the same subject!
Prepare a lesson which will serve either as a revision of a familiar topic or as an introduction for the first time. Make sure that any questions you are planning to ask have follow on questions that probe more deeply if the students can glibly rattle off the answers.
As interview season comes round again, I was thinking about how many 'sample lessons' I have seen over the last few years in my capacity as a middle-manager.
And how many perfectly decent applicants have shipwrecked themselves in 20 short minutes by completely missing the mark in this brutal but necessary ordeal.
If that sounds like way too much to cope with in the time you have, you are probably being much too ambitious in the material you hope to cover.
notice if you are pitching the lesson at the wrong age group.
 I would say, as a general rule, we are looking for some up-front teaching to prove you can be interesting, hold a class's attention and explain your subject.
But also we need to see one or two good student activities to prove there's more to you than just chalk and talk.
Make sure all your activities are that little bit special, so that even if they have bashed through the subject before, you are giving them an interesting new take on the topic. A matching exercise with challenging distractors instead of a simple fill-in-the-blanks.'Please write your names on the cards and put them in front of you.'After spending 3 minutes doing this, he taught for 20 minutes and never once referred to any of the students by name.