Being able to take a screen shot is a very important skill for teachers.
If inappropriate material appears it really helps if teachers can illustrate the material to facilitate the process of blocking the site quickly.
In the event of an error having a screen shot allows the IT help technicians to focus on the exact problem which helps to speed up the process of repairing the problem.
If you are using a Windows machine:
Print screen Capture the entire screen
Alt-Print Screen Captures the active window to the clipboard.
Copy the image into Paint or any image editing application.
If you are using a Mac:
See: Built in options