Electronic letters are sent via e-mail and tend to be very brief. It is important to include the position you are seeking in the SUBJECT of your e-mail. Then in the BODY of your letter, sell two to three points about yourself that will demonstrate that you have what the employer is looking for.
Dear Principal Name, I wanted to let you know that my child, name, will not take part in the name the test this year. We ask that you make arrangements for him/her to have a productive educational experience during the testing period. Thank you for all you do.