Fair Use is a flexible exemption to copyright that balances the author's need to make money with society's need for knowledge to be shared and reused.
Not all educational uses are Fair Use.
Fair Use is determined by weighing four separate factors and looking at the combined outcome of all four. All four don't have to be favorable, but as a whole, they should be more favorable than not. In other words, you can flunk one factor and ace the other three, and it still might be fair use. Or you could do so-so on all four, and it could still be fair use.
A few things are always considered Fair Use:
Transformative Works are always Fair Use, because of the unique way that they fulfill the criteria of the first and fourth factors.
A Transformative Work is not a derivative work, although it might have started out that way. But instead of being a new spin on the original, or the original with something added, taken away, or changed, a Transformative Work is something entirely new that just uses elements from the original. It repurposes, recontextualizes and/or changes the work from which it borrows.
Transformative Works pass the Fair Use first factor test (nature and character of the use) with flying colors. That is because a Transformative Work creates new ideas and knowledge, which the courts consider socially beneficial enough to fall under the Constitutional phrase, "to promote progress of science and the useful arts."
Transformative Works pass the fourth factor test (market effect) equally well. That is because a Transformative Work is something entirely new that can't serve as a market substitution for the original work.
Examples of Transformative Works that have been accepted by the courts as Fair Use:
If you're interested in this topic, you'll want to watch these two features about transformative works and remix culture: