Such a school is not completely educational (4)

I believe the answer is:

prep

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'such a school' is the definition.
(prep school is a kind of school)

I don't understand the rest of the clue.

(Other definitions for prep that I've seen before include "Homework (abbrev.)" , "Homework (abbr.)" , "warm-up race" , "School work done outside lessons" .)

I've seen this clue in the Evening Standard.

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