Both Uses of

principal
in
Arrowsmith

The hall was a third full of summer students and teachers, and men who might have been doctors or school-principals.†
Chpt 17 *

principals = most important; or people that are most important

He was summoned into the reception-room of the department to explain to angry citizens why the garage next door to them should smell of gasoline; he went back to his cubbyhole to dictate letters to school-principals about dental clinics; he drove out to Swede Hollow to see what attention the food and dairy inspector had given to the slaughterhouses; he ordered a family in Shantytown quarantined; and escaped at last into the laboratory.†
Chpt 21

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.