To leave at the end of December or April you need to give two months notice. So the chance to resign for then has passed. For the summer, you need to give three months notice, so resign by May 31 at the latest. These dates were set before the six-term year became common. You could be required to work until the last day of your notice. If you are going to a new school, most heads understand your need to be present at the start of term in the new school and to have a holiday in between. Tell your head about wanting to move as soon as you start looking. You will be giving the school as a reference and you don't want them to hear that way.