The Copyright Office has laid out a few guidelines that sort of set the minimums and maximums of what counts for educational fair use, if you are on the educator side of the bracket. You can read them all in the recommended (and not, relatively speaking, overly long): Reproduction of Copyrighted Works by Educators and Librarians (pdf). Down in the section titled "Agreement on Guidelines for Classroom Copying in Not-For-Profit Educational Institutions with respect to books and periodicals" (page 6 of the document), you will see some specific amounts set forth for teachers making copies for class (or for those making copies on behalf of a teacher's lessons):
If you notice, this still fails to go into details about what a student can use. The answer? At least the best answer you are going to get? Look to the four watch words above. Are you quoting/referencing short, necessary bits in a paper that will only be seen, at most, by your professor and classmates? Then generally speaking, fair use will cover the majority of what you do.
When in doubt, see: Checking the Copyright Status of a Work (pdf), and keep the use the material in question down to what fulfills and enhances the assignment. Do not quote a page when a couple of lines will suffice.
Note: attribution is not enough to waive aside copyright infringement. That's more or less a direct quote from the same "Fair Use" document linked to above: "Acknowledging the source of the copyrighted material does not substitute for obtaining permission." If you are going behind a short quote/paraphrase, go ahead and try to get permission.
Here are 8 tips that will help you with fair use in the classroom.