When you've put in all the work of creating something, you should have the right to have at least some control over how it gets used, right?
That's the basis of copyright -- to create an incentive for creative people and businesses and organizations to keep creating, because they get credit for what they've done (and maybe even payment, if that's what they decide).
Copyright law makes some exceptions for this, called "Fair Use", which allow you, while you are a student doing classwork, to use copyrighted material, within limits.
A special thanks to Dennis Van Arsdale, Technical Services Librarian, Boreham Library, University of Arkansas - Fort Smith for sections of this LibGuide.
Here are some basic rules to follow that should help you handle copyright legally in the classroom. Remember: these rules do NOT apply outside your classwork -- they are much more strict outside the classroom.