That's an interesting example because it illustrates out how fair use is in part defined by its effect on the market.

A teacher copying a New York Times article verbatim and distributing it to a classroom for the purpose of analysis or criticism is likely to be held by the courts as fair use. On the other hand a teacher photocopying an entire textbook (or distributing a PDF) would probably not be considered fair use.

Both are copying educational material in full, but they have a different impact on the market. Distributing copies of an old news article to 30 students realistically isn't a direct substitute for 30 subscriptions to The New York Times. But a textbook publisher's business model specifically revolves around selling its textbooks to students - the teacher is distributing their copyrighted material to their target market and removing any incentive for the students to pay the publisher.