Resigning from a school is governed by law. Technically, a school is ruled by the same workplace laws as other institutions. This means that you could give a month’s notice, and at times this may be necessary, depending on your situation. However, this is not the accepted etiquette in a school. You should expect to give a half term’s notice at least. This means to leave at Christmas you must resign by the end of October half term; to leave at Easter, by the end of February half term and for the summer, by the end of May half term.
This means you need to start looking for your next job relatively early. This is one of the few professions where you might start looking for a job potentially seven months before you take up the position. Before you start applying, it is the etiquette to inform your line manager and your head teacher. The school is required to release you for interview and respect your privacy with classes about your desire to work somewhere else.
The school cannot seek a replacement for your post until you hand in your official resignation letter. Just telling the head teacher that you have been offered and have accepted a new job is not enough to indicate you are resigning. You have a right to change your mind up until the point you hand in a letter. Therefore, you need to send this to your head teacher as promptly as you can.
You are within your right to wait for confirmation of your new post in writing. However, the longer you leave resignation the more pressure you place on your senior leadership team to replace you as a teacher. If you are feeling antagonistic to your school leadership this is obviously a means of showing them your displeasure. You could indeed wait up until the deadline. However, remember that there are students who need great teachers when you leave. Therefore, hand in a timely resignation letter for these children at the very least.
When you resign, you should indicate to your line manager how you would like the news of your decision to be handled. You are going to continue working in the institution for a good amount of time after resigning, therefore you are within your rights to control the flow of information to colleagues and to students.
For most, resigning from a post is sad but part of a natural progression of a career. It likely will not come as a surprise to your manager and a lot of the advice given here is merely fulfilling the etiquette and the legal requirements to help your post be filled promptly.