Fair Use allows limited use of copyrighted materials for the purposes of teaching and research without getting permission from the copyright holders. It doesn't mean that all educational use is considered fair use.
To think through whether a particular use is a fair use, you have to look all four factors and no one factors is decisive. As stated in U.S. Copyright Office, Section 107 also sets out four factors to be considered in determining whether or not a particular use is fair:
There is no specific number of words, lines, or notes that may safely be taken without permission. Acknowledging the source of the copyrighted material does not substitute for obtaining permission. The safest course is always to get permission from the copyright owner before using copyrighted material. When it is impracticable to obtain permission, use of copyrighted material should be avoided unless the doctrine of fair use would clearly apply to the situation.
Disney Parody explanation of Fair Use.