Both Uses

preparatory
in
Ferris Beach
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I felt Misty's absence stronger than ever, and I kept thinking of things I needed to tell her, how Mrs. Poole had spent thirty stupid minutes talking about little Ruthie Sands going to prep school so as not to have to participate in integration, how I had heard the DJ on the radio say that they were going to open a new dedication line, how sometimes the thought of walking into that brand new high school made me feel so scared I felt sick, and how she was the best friend I had ever had and ever would have, and how I would give anything in the world if her mother had not left home.†
Chpt 17
He prefaced everything with "Nobody in Greensboro would do that," or wear that, or say that; he called Fulton "Gritville" and said he couldn't wait to go off to prep school the next year.†
Chpt 18 *

Definitions:

(1) (preparatory)
done to get ready for something that will happen later
(2) (meaning too rare to warrant focus)
The expressions preparatory school and preparatory classes typically imply preparation for college. Often, preparatory school is shortened to prep school.