How to write an apology letter

The Principal,
YYYY Secondary School,
1, Portland Street,
Mong Kok, Kowloon.
12th June, 2011.

Dear Mr.\ Ms. XXXX,
Re : Letter of Apology
I write to apologize for being late at school yesterday because I overslept for more than 20 minutes.
In order not to go to school late, I went to school by taxi.
However I was late. I understand it was my fault for not getting up early but I am sure it might not happen again.

Finally I should be grateful, if you would kindly accept my apology.


Yours respectfully,
zzzzz