If a work is is in the public domain, anyone can use it freely, but you should still provide proper attribution. Works may be in the public domain due to their age, status as U.S. Government documents, or because the author has relinquished their copyright. Copyright terms for the United States and other countries are often different, and the time at which a work enters the public domain may differ based on when and where it was created. These charts specify when various types of works enter the public domain.
In general, these works fall into the public domain:
Do not just assume that a work is in the public domain - be sure to check. If the work is not in the public domain, check to see if your intended use falls under either the education exception or Fair Use. Lolly Gassaway's When U.S. Works Pass Into the Public Domain chart can help you determine whether works you want to use have passed into the Public Domain.
A lot of public domain information exists on the Internet that you (and your students) can use.
Several tools can help you check the copyright status of works that you want to use. If it was created after 1978, it is best to assume that the work is protected by copyright.