Dear RGP (Really Great Principal):
We have worked together in some capacity for 12 years, years where you gave me so many opportunities and advice. You helped me develop into the teacher I am today. I cannot describe how much I am going to miss seeing you every day. While I am truly excited to tackle this new challenge, I am really sad to leave. I want to thank you for all you have done, and all you haven't known you've done.
P.S. You should have know when you told me to "blog it", I probably would do just that.