Speaking from the viewpoint of an ex-supply teacher who routinely stood in for those who were signed off, two things spring to mind. 1. The repeated fortnightly sick note solution works - I stood in for somebody in exactly that position until she decided to call it a day, and her resignation was approved well within the usual notice period. 2. I have stood in for teachers who have literally had their contracts ended overnight by the HT at their discretion, when it has suited them to do so. The notice period can be waived in certain circumstances.