You may of course copy material that's in the public domain either by original creation or by expiration of copyright. In addition, "Fair Use" governs how you may copy and redistribute copyrighted information in an educational setting. Here are the criteria the 1976 Copyright Act set forth to determine whether use is fair or not:
U.S. Code, Title 17, section 107:
"In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include--
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole
(4) the effect of the use upon the potential market for or value of the copyrighted work.