I approached my manager around this time of year and said, "What do you think the value of an MBA or MSF or some advanced degree would be for me on my career track?"
Sort of approach it like you're going to them for career advice, and they'll probably be a little flattered and happy to give you their advice. Just make it sound like it's something you're thinking about, not something you're definitely going to do. So after you get their advice, you can return to them a month or two later and say, "You know, I've been thinking a lot about going back to school, and I've been thinking about what you said, and I'm leaning towards business school. If I end up applying, would you be willing to write a recommendation?"
So kind of ease them into the idea of it, rather than spring it on them like, "Hey, I'm quitting in a year. Can you help me get into school?"