Fair use is an exception and a privilege. It is not a right. Fair use is also not automatic protection against litigation. Certain quantities and percentages exist as guidelines to determine whether something is allowable under fair use, but there is no exact quantity or percentage of a work that will automatically qualify its use as fair use. Fair use is always a judgment call and it is dependent on multiple factors. The person responsible for sharing the work is the person responsible for the legal violation. In cases where an individual is employed by an institution, the institution is also usually implicated in the case.
Just because something is used for educational purposes does not mean that it meets the criteria for fair use. Individuals and universities can and have been sued for cases in which they believed they were compliant with fair use guidelines.
In order for work to be shared under the fair use exception, the use must comply with four factors that take into account the nature and amount of the work being used and how that work will be used. Below is information on these factors, some resources to assist you in determining whether the fair use exception may be applied to your specific case, and additional information on fair use and copyright.