I agree that many observations are such a limited snapshot view that they aren't fair, but the latest ofsted criteria does include prior data as a key indicator of performance. It should not just be a 20 minute exercise in ticking boxes and this does alleviate the issue somewhat, if your data is good and shows progress then you should be credited for that. One key thing we're constantly hearing from ofsted and other observations is that the best lessons have minimal (approaching no) teacher led learning, all of it is students doing the learning, you're there to address misconceptions. While this is fine for observations and such, as a long term teaching strategy for maths it is not feasible, even the brightest need "chalk and talk" from time to time. I had an observation yesterday with my year 10s and was graded good, which is good enough for me. They weren't an easy group when I got them and still aren't, yet I know they are making good progress and will get the results they deserve at the end of the day.