There's a warm glow of satisfaction that you get when you've taught a really good lesson. This is what teachers love about their job, isn't it? That light headed feeling of elation - if I remember rightly. Everything seemed to fit into place and everything felt so I spontaneous. The only trouble with spontaneity is it needs such a lot of planning!
As a new teacher, one of my biggest fears was running out of things to do. This actually happened when three of the biggest "pains" were absent and the rest of the class sailed through all the work I had planned. It left me with 20 minutes to fill. You should put approximate timings for each activity into your plans and include extra work for contingencies.
Extend the more able. When setting classwork, bear in mind the needs of the really clever pupils and the faster workers who may finish well before everyone else.
Set "extension work", which is work that you do not expect everyone to do but should be of benefit to those who do attempt it. Extension work should be valid, relevant and demanding. Try to avoid giving more of the same kind of work. You should also avoid starting pupils on the next topic, which you will want everyone to start together as a whole class.
You need variety, and you are much more likely to keep your pupils motivated and on-task if you break up the lesson into a range of different activities. These could include class discussion, small-group work, written exercises and so on.
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