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. Shareware is usually distributed freely at no cost for a trial period. The software contains registration information and if a user intends to keep using the software they are expected to register with the distributor and pay a fee. The fee is usually considerably less than regular software.