Dear Mr. Smith,
Thank you very much for giving me so much time for last week's interview.
I was very exciting you could provide the position for me as I'm a student who just graduate from college now. But I must say sorry for your believing in me. I could not be able to take this job. Because I have passed the examination to graduate school. As I have mentioned to you at the interview that I have attended the graduation exam half a year ago. At that time,I don't know the results. But now I received the acceptance letter from the school. So I couldn't be able to join in your company. I'm sorry for it. But here,I still want express again my deep gratefulness to your job and warm welcome. I hope and am sure we will have opportunity to cooperate in the future.