Teachers really like the ability to display their iPad or their students' iPads on a projector screen. Projecting on a large screen is great for demonstrations, simulations, explanations, and showing examples. There are several ways this can be done in the classroom.
If you don't mind keeping your iPad in one spot, then a VGA adapter (for 30-pin Dock connector or for the new Lightning connector). Apple has made four types of adapters – determine which one you need. Or, a document or USB camera might work for you.
If you want to wirelessly transmit your device's screen and audio so that you or your students can walk around the room, then it gets more complicated. You'll tap into Apple's AirPlay feature that is built into all iPad 2s and newer, including iPad mini. AirPlay works over Wi-Fi and requires all devices using it to be on the same network.
Apple TV is a small black box that can connect to a projector. iPad can mirror wirelessly to Apple TV using AirPlay. Apple TV only outputs HDMI. Your projector might not have HDMI input. If that's the case, you'll need an HDMI to VGA adapter like the Kanex ATVPRO.
If you already have a computer connected to your projector, you should look into using software to turn that Mac or Windows PC into an AirPlay receiver. You can download and try for free AirServer, Reflector, or X-Mirage. The software runs on your computer and allows devices to mirror iPad to the computer screen. Since the computer is connected to a projector, then the iPad shows on the projector. I've written lots more about both AirServer and Reflector.
In my visits to various schools, I'd say that Apple TV, AirServer, Reflector, and X-Mirage work about 50% of the time. That's because AirPlay requires the device and the computer or Apple TV to be on the same network. Often schools have different networks for mobile devices and for PCs so AirPlay won't work. Also, AirPlay requires specific ports to be open on the network and frequently they are not. It's a good thing there are free trials of AirServer, Reflector, and X-Mirage so you can test your school's setup before purchasing.
If your Wi-Fi network won't cooperate with AirSever or Reflector, you should check out iTools. It's free software for Windows PCs that will mirror to a computer using your device's charge/sync cable. Read more about iTools.
I've whipped up a chart that compares these methods of displaying iPad's screen on a projector. You'll want to click the image to download the easier-to-read PDF version.