The actual subject/content of the lesson isn't so important, but you should choose something you feel confident in teaching - something where you know you have the answers to any surprise questions the students might ask.
The observer will be looking for lots of things, such as:
A good warmer - get the lesson off to a lively, snappy start.
raport - make the students feel comfortable and relaxed, so that they have fun during the lesson.
pace - keep the lesson moving along, but don't rush through an activity to get it out of the way (this is a mistake I made in my last observed lesson).
monitoring - move around the class a lot. Don't stay at the board.
groupings and student interaction - try to have some whole class teaching, an activity where the students work alone, some pair work and some group work.
Also - try to include some controlled practice and some freer practice.
Continuity - it should be clear to the observer how each part of the lesson feeds into the next. Try not to have any activities that are just slotted in to fill time. Each part of the lesson should contribute towards your main overall aim.
These are just some things off the top of my head - I'm sure other teachers can think of things I've missed.
Brian
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