Contact personnel concerning the company's leave-of-absence policy before you speak with your manager. Find out what the time limit is for a sabbatical. Companies usually authorize LOA's for the following reasons: illness or injury, maternity, military service, and education. Initially, you can tell them that you are considering going to school. It's best to take an LOA, even if you will be gone longer than the allowable time, in case you must return early. They may hold the position open for you, even if you are a few months late, as long as you stay in touch.
Beware of announcing a resignation too soon. Bosses are famous for major changes in attitude, and also finding your replacement quickly. Besides, you are never really sure that you are leaving until the last few months before departure. There are many last minute hitches or nerves that could stall or prevent your trip.
For something different, check out "Six Months Off: How to Plan, Negotiate, and Take the Break You Need Without Burning Bridges or Going Broke" by David Sharp, James Scott, and Hope Dlugozima.