The easiest way to mirror is to attach your iPad directly to your computer. Show your presentations, videos, and even games without a hitch. It is a very useful way to display your iPad to a large group of students/colleagues. Once set up, your device that you control will be displayed.
Mirroring your iPad/iPhone and even the latest 5th generation iPod touch means showing on a second screen everything that is shown.
Reflector ($12.99, formerly known as Reflection) is a great Mac/PC app that is extremely easy to set up and use. Here is an affordable way to mirror your iPad/iPhone in your classroom if you do not wish to purchase the $99 AppleTV.