The exact meaning of principal can depend upon its context. For example:
"the principal idea" or "the principal performer" — the most important
"the principal of the school" — the person in charge
"a principal of a company" — an owner or person with controlling authority
"the principal in a transaction" (as contrasted to the agent) — the person on whose behalf an agent acts
Then, in a booming voice, Jillson announced, "And now, everyone, it is our great pleasure to introduce to you the esteemed founder, president, and principal of our beloved Institute: Mr. Ledroptha Curtain!"
There are no more uses of "principal" in The Mysterious Benedict Society.