Carefully check the fine print that the union provided about any window period during which they will allow you to cancel. Often it is 15-30 days before the anniversary of the date you signed their membership form. Prepare the letter and put the first acceptable submission date on your calendar.
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.
Dear (union president's name): Please accept my resignation from the postion of (whatever it is), effective (date). Just like any other kind of resignation, it's best to keep it completely polite and very simple: details are not necessary.
Dear Union: I, insert resigning member's full name notify the insert union name, herein UNION, and my employer, insert employer's name, herein EMPLOYER, of the following: I hereby resign my union membership, from my local and all of its affiliates, effective immediately.
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.