A principal is a person who is at the top, or a person whose has the foremost responsibility for an organization. It is often associated with schools. In the United States, a principal often refers to the person in charge of a grade school such as an elementary or high school. In England, it can refer to the head of a college or a head teacher.
My head teacher cancelled the school concert after finding out that the band director had been rushed to the emergency room only hours before.
Mrs. Kelly dreamed of being a high school principal in sunny California but here she was, a classroom teacher in a small primary school instead.
A principal can also be a key player, or important player in a group. For example, a symphony's principal violinist would be the most talented or most vital violin player.
Since one of the principal actors were constantly ill throughout the season, the show closed weeks earlier than had been expected.
To fail is to not succeed, to not reach the goal you had in mind or do the thing you wanted to do. In school, it means to not make a passing grade.
Although Regina failed as an artist, she found success in a career as a curator of an art museum, and found that she was just as happy in that position.
When George failed two of his university modules, he decided that engineering was not the right degree for him. He decided to try agriculture, as he loved farming.
Fail can also mean that you did not do something that someone else expected you to do, usually in regard to common courtesy.
Christopher failed to keep his appointment with the dentist, or to call to tell them he could not make it, so he was charged for the time he was not there.
Laura, you failed to inform me that you were going out tonight. I made your favorite dinner especially for you.