I am not sure what maarat ayin translates to, but I am guessing from the context that you are creating a situation where you are considered to be ‘evil’ for your wrongdoing and that you are not revealing the source of the discontent.
You clearly have no obligation to disclose your specifics if you are late to a meeting of a class, but to offer the participants who await for you a breif explanation that you were involved in another matter is a simple courtesy, if they seem to expect some explanation as to why you might not regard their time and commitments with a priority. But that said, this is a minor minor topic and I can assure you that your future in the world to come is not dependant upon how you present to your class where you have been but rather it is based on your total disposition in giving and caring for others. Best wishes.