In the Face-to-Face Classroom: The Classroom Use Exception
When the Classroom Use Exception applies to your situation, as it does in only limited cases, it is delightfully clear regarding the rights it gives you.
To qualify, you must be teaching face-to-face in a classroom setting at a non-profit educational institution. In such a setting, the exception gives you the right to perform or display any copyrighted works at any length. Examples of the kinds of activities this would permit:
Displaying images on a screen
Passing a book around
Singing a song the students know
Playing a song
Showing a movie
Remember that this exception does not permit copying or distribution. Remember that copyright is actually a "bundle" of rights. This exception gives you the right to use just two of the rights in that bundle: performance and display. This means that the classroom exception does not give you the right to make copies and hand them out or, indeed, to make a copy of an image in order to display it.
It is also very important to remember that this exception only applies to teaching that happens in person in a classroom setting. It does not apply to course websites, synchronous or asynchronous online instruction, or any other mode of instruction where all of the participants are not joined together in person in a classroom (or classroom-like) setting.