When should I do a risk assessment?

All lessons!  Don't assume that if there is not a practical activity, there is no risk involved. Think carefully about the resources you use, the laboratory conditions and facilities and the pupils you teach!  If you are not entirely happy with the health and safety then report it ...before the lesson starts.

For all practical activities, including demonstrations, a Risk Assessment, is absolutely essential. There should be evidence that you have done a risk assessment, before the lessons starts.