I had this same concern, especially since I don't necessarily work in a field where it is expected you will get an mba. I just talked to both my supervisors last week and it went well. This is pretty much exactly what I said "I've wanted to go back to school and had been thinking about ASU (my undergrad and local university) part-time mba and most likely that is where I would still go, however as I've been researching further, I think I may actually have some shot at a top 10 school. I know it's only like a 15% acceptance rate and it's a long-shot, but I figured I owed it to myself to at least try for an amazing opportunity. I really do love working here and of course wouldn't leave unless it was for a once in a lifetime opportunity that I just couldn't pass up, that's why I'm only applying to ASU and a few very top programs. This is something I'm really excited about and hoping you would help support me by giving me your recommendation. This wouldn't be until a year from now if I got accepted, and I would know either way long before that, so of course you would have plenty of notice of my plans."