This time around we started Unit 1 in the summer term of Year 9 and did the exam in November Year 10. Unit 2 we will do in June of Year 10 and then Unit 3 will be in Summer Year 11. I think most schools though (and we will be following them next time around) do Unit 2 first, then Unit 1 and then Unit 3. The advantage of doing Unit 2 first is that (almost all) of the number work in Unit 1 (calculator) is covered in Unit 2 (non-calculator). Doing unit 2 first and teaching all the number work without a calculator would have a very big advantage then time wise when it comes to doing Unit 1, i.e. these are the same topics we have already done kids, just use your calculators this time! Hope that helps.