I've seen it done several times. When you know in advance that you will be moving on, your advance notice of resignation is a gift you give to them. More importantly, you do bypass the questions about resigning to avoid termination. I've been sick, and not at my best. Tulipteacher nailed it, and it allows you to honestly answer "no" to all those tricky questions. Once again, I would recommend talking to your union rep if in doubt. You are giving the district the gift of time - time for them to find their "perfect" candidate, time saved by not being involved in whatever turmoil is going on, and the assurance that you will not share tales to sour the workplace culture more than whatever currently exists. By giving them this head start, you really don't need much of a reason. Change of life plans would be as specific as I would get, and it would be what I say to anyone who asks. That could mean going back to school, taking a new job, teaching at a college, moving out of state, and the list goes on. They will be grateful for the lead time when new graduates start sending in applications, maybe getting you less scrutiny. You do give up unemployment benefits, but those would only come to you if you wait to be non-renewed.