The four factors of Fair Use can help you determine if and when it is ok to copy content for teaching and learning and in the classroom and in the campus library. Those four factors are:
No one factor is more important than the other and each use must meet all 4 factors. So the educational use, alone, does not qualify something as allowed. And the amount of a work used does not qualify something as allowed. Fair Use is a balance of all four factors.
There's no one right answer as to what constitutes a "fair use" of a particular copyrighted work. The answer varies from situation to situation.
A good point to consider is this: Have you made a "good faith" effort to comply with the "fair use" clause of U.S. Copyright Law?