It sorta went like this:
https://www.youtube.com/watch?v=hTOKJTRHMdwBut all jokes aside, just thank them for all their help and guidance and assure them that you will do whatever it takes to facilitate a smooth transition. I am leaving in early May and my boss was grateful for 6 weeks notice. If they genuinely care about you or your career, they should be happy for you and wish you luck.
As an icebreaker though, I said "I have good news and bad news...the good news is that I was accepted to an MBA program, the bad news is that its a full time program". (Its sort of casual between me and my managers so you may not want to take this approach.)