Your career comes first. As a conscientious teacher you want to do the best for your school so try either to change the date or the time of your interview. If part of the interview day is spent teaching a demonstration lesson, offer to do that on another day. At the least, you will appear as a committed teacher who isn't trying to leave your present school at any cost.
If the interview can't be changed, you will have to weigh up loyalty to your present head against how much you want the new job. If it is the post of your dreams and you are likely to be the best candidate, attend the interview. If it is a long shot, you might feel differently. Only you can decide.
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