Fair use is an exception to copyright law. It permits certain limited uses of copyright materials without the need to ask permission. All that's required is a careful analysis of the use examining each of four factors. Not all four factors need to be met in each case. But the direction of the analysis should weigh in favor of fair use for you to proceed.
A fair use analysis should be carefully conducted for each use of copyrighted material. A handy checklist has been created by Columbia University to help in the analysis.
"Amounts of material used for online course support should be tailored to the educational purpose, though it will not infrequently be the case that access to the entire work (e.g., an illustrative song in a class on the history of popular music) will be necessary to fulfill the instructor’s pedagogical purpose."
-Code of Best Practices in Fair Use for Academic and Research Libraries (2012)
The 10 percent rule is part of a set of guidelines agreed upon by educators and publishers that never became law. It can be a useful guide to determining if an amount exceeds fair use, but one should bear in mind that to exceed 10 percent won't automatically mean that courts will look askance at the use. Neither will using 10 percent or less always guarantee that an instance is deemed fair use. Instead, a careful fair use analysis should always be conducted.
Increasingly, when looking at the four factors of fair use, courts are prioritizing the first factor, with regard to what is known as a transformative use. What is a transformative use? It is alteration of the original work to new purpose and new meaning. More precisely, "transformative uses are those that add something new, with a further purpose or different character, and do not substitute for the original use of the work." A classic case of transformative use is parody; even though a significant portion of the original work is used, the character and purpose is dramatically altered.