Legal and ethical behavior is an essential component of being a good digital citizen. A key factor of that behavior is knowing what content is and is not okay to use in an educational setting. For answer to your questions and information on the legal doctrine called "fair use," visit the following web sites.
Copyright and Fair Use in the Classroom, on the Internet, and the World Wide Web
Copyright Website
What is Copyright Protection?
Copyright and K-12: Who Pays in the Network Era?
What is Plagiarism?
Turnitin Research Resources
Plagiarism.org
Avoiding Plagiarism