The most common problem is that teachers buy a single-user license, designed to be used for a single computer, and then shares that software with colleagues. That is illegal and can result in punitive action at a local, state, and federal level. Many students are very savvy computer users and enjoy showing off their skills to their friends.
Students can cause tremendous havoc in the classroom by changing settings on the computer etc.
One way to handle this situation is to have a strong Acceptable Use Policy in place that is both easy to understand and enforceable. No legal obligation to purchase, but an ethical obligation to reimburse the person that designed the software. US Copyright Office a good overview of basic copyright law. This is a one-page overview of copyright issues for teachers. Ameritech's Cruise Control Web Site Cruise Control is a starting point for parents, teachers, kids and teens to become sensible, safe internet surfers.