you have been requested to judge a one act play competition however you are unable to do so due to unavoidable circumstances. Write a letter of regret to the principle of host school declining the invitation

You can expand the paragraph given below as the body of your letter:

I regret to inform you that due to some unavoidable circumstances, I will not be able to accept your invitation. I thank you for your kind invitation and deeply regret the inconvenience caused.. (continue based on your discretion..)