Unless educational materials designers and vendors are able to earn a profit making and selling these materials, there will be fewer and less quality materials available to you for your classroom. Finally, you are setting an example constantly for your students and you should display the highest ethical standards. Almost all materials which might be used by educators are protected by the copyright law unless it is specifically stated in the work itself that it may be copied and used without permission. The elements which figure in determining fair use are complex and subjective. Finally, never distribute materials which contain any questionable material, even to other teachers or students for free.
Computer software requires some separate considerations.
When you purchase software you purchase a license to use that software. Your license may be for a single copy of that software to be used on a single machine. Schools also purchase software for distribution on a server on a network. In that case they may purchase only a single copy, in which case only one person should use that software at a time. If they purchase multiple copies, they must monitor how many people are using the software at a time unless the number of licenses cover all of the machines on the network.