You have to be able to touch, see, or hear the work. Books, Poems, Photographs, Musical Recordings, Performances, Sculptures, etc are all protected the moment they are created.
There is something called "Public Domain". Anything from the public domain is no longer under copyright protection, and is free to use! (Usually because it is older, created before 1923)
You can use anything you (or your school) have purchased rights to! The purpose of the Copyright Act was to protect the creators of works and to promote new ideas, something our whole culture benefits from!
In a word: ASK. Teachers do have more leeway with Copyright regulations than the general public. But, just to be sure, before you use something in your classroom - make sure you ask permission from the copyright holder!
There is text built in to the Copyright Act that speak about "Fair Use". It gives the pubic the right to do limited things with copyrighted work for the use of criticism, news commentary, and education among other things.
Whether or not something is considered "Fair Use" is up to a judge. In a court. Who could send you to jail. They are asked to consider four criteria when deciding whether what you did falls under "Fair Use" or "Copyright Infringement". (To avoid the judge, just ASK PERMISSION beforehand!)