Is it considered a sin if a teacher does not pay the penalty clause agreed upon with the school, which resulted from her leaving work because her husband swore an oath of divorce, knowing that she is able to pay it?

There is no sin upon you for not paying the penalty clause fine, as long as you are excused for leaving the job. It is stated in the resolutions of the Islamic Fiqh Academy: "The penalty clause is not enforced if the party against whom it is stipulated proves that their breach of contract was due to circumstances beyond their control, or if the party for whom it was stipulated proves that they did not incur any harm from the breach of contract."