Both Uses of

principal
in
1st to Die, by Patterson

They looked like a bunch of street-struttin' kids auditioning for the Grammys, but the principal swears they'll come out of it eventually†
p. 121.4 *

principal = most important; or person that is most important

Not unless they've got a bleeding kidney, which to my knowledge, none of our principals show any signs of.†
p. 387.8

principals = most important; or people that are most important

Definitions:

(1) (principal)
most important; or person who is most important
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
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

See a comprehensive dictionary for specialized senses in finance, architecture, law, chemistry, and music.