Teacher's punishment is starting to trend - dividing the foreign school (5)

I believe the answer is:

lines

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'school' is the definition.
(thesaurus)

I don't understand how the rest of the clue works.

(Other definitions for lines that I've seen before include "Skin wrinkles" , "old-fashioned school punishment" , "form of school punishment" , "Ranks" , "Wrinkles; rail tracks" .)

I've seen this clue in The Irish Times and the Irish Times (Crosaire).

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