To dream of a school principal represents social justice. A principal may be an ability to embarrass someone, make them insecure, or control what they think. He may also reflect an unpleasant issue in your life that you feel powerful enough to ignore, or manipulate in your favor.
A school principal may also represent your feelings about an issue that embarrasses you, or forces you to do something you didn't think you had to. You may have thought you could get away with something that proved wrong. You may feel guilt or regret.
To dream of an evil principal may reflect your own insensitive use of power to embarrass someone, or your projection of an insensitive situation that feels embarrassing or unfair.
To dream of fighting or attacking a principal may represent your defiance or resistance against an issue that you thought you could get away with or never need to confront.