Both Uses

principal
in
Roll of Thunder, Hear My Cry
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Consisting of four weather-beaten wooden houses on stilts of brick, 320 students, seven teachers, a principal, a caretaker, and the caretaker's cow, which kept the wide crabgrass lawn sufficiently clipped in spring and summer, the school was located near three plantations, the largest and closest by far being the Granger plantation.†
p. 15.9

principal = most important; or person that is most important
"Hey, Cassie," said Mary Lou Wellever, the principal's daughter, as she flounced by in a new yellow dress.†
p. 17.1 *

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