Would you NOT tell your employer about leaving for bschool?

Be honest. If you're leaving the industry you're in now, you must have a reason for doing so, right? Isn't it some reason of personal value? Your supervisor should understand that. Like he said, you're young, so wanting to change your career should not come as a surprise to him. When you do have the conversation, just be frank about your goals in life, and why an MBA is more important than the job... or just say why the MBA is important, in case you do end up staying.

How to bring it up? Ask him to do a 6 month/12 month/18 month whatever review. Be prepared. This is what I did and it really helped as I was able to talk my boss through all the recommendation questions before even telling her why my questions were so specific. Guide him through the conversation. Rate and describe yourself, then ask for his feedback on that. Basically walk him through a recommendation letter, and give him content for it, too. At the end of the conversation, talk about your goals. You're not leaving tomorrow, so you should have goals for when you're at the company and in the future. The future plans would involve getting your MBA. Then let the conversation flow naturally.

When I did this with my supervisor, I had a six-page document that touched on every point in the recommendation letter, with details and examples to support it. When I told my boss I wanted my MBA, she asked to hold onto my writeup (all part of the plan). This not only made my goals clear, but it gave her material for the letter she wrote.

Good luck!