I'm a Doctor, atleast I play one on the internets.

Principals are incharge of schools. Generally the main person or sometimes the main thing in some process.

Principle is a type of concept or theory. Most people confuse "principal" with "concept" and call the main theory components principles.

In Wing Tsun we say that Principles are not stand alone theorys but a group of concepts that work together to make a system. Concepts are the stand alone theorys that are more universal.

"Always go forward" is a WT principle that only works in conjunction with sticking, yeilding, and following. This makes a system that we use, they do not stand alone.

"Balance" is a Latosa Escrima Concept that applies to all MA and contains the above system of principles. It can be used as a stand alone theory.