First of all, good luck! We all hope you get the promotion.
When you write the letter, remember to mention what you have done for the school to earn a promotion. For example, write about how well your students did in the next grade, extra projects you took on, or extracurricular activities you supervised. Write about why your promotion is good for the school, not yourself. If you can �t think of reasons the school should promote you, maybe you need to make some reasons and ask later.
Compose your letter, and check it carefully for any mistakes. If you can, get another person with good language skills to look over it too. One mistake might mean no promotion.
You may want to use one of the business letter templates in Word. You can adapt it easily if there are details you don �t like so well or if you don �t want it to be obvious. Use a common font such as Arial or Times New Roman. Most of the common email fonts would be just fine, but not Comic Sans or Courier New. Make sure the point size is large enough to be easily read, but not so large that you need two pages. Also, since you say letter and not email, be sure you use nice paper and envelope, just as if you were sending in a paper resume.
Again, good luck! I hope this helped.