The simplest way is to plug the iPad into the projector using an adapter and video cable.
There are a few drawbacks to this setup. The first is that you are tied to the projector. You can get cables of various lengths, but you are still tied to that cable and can't wander around the room. Secondly, if your classroom computer is also connected to the projector then you'll constantly be disconnecting one device and then another.
If you have audio from your iPad you want to play, you'll also need an audio cable from the iPad headphone jack to your classroom speakers.
However, despite those drawbacks, this method is very reliable. Since you are connected directly to the projector you don't have to worry about the reliability of your wireless connection.
http://suprtektalk.blogspot.com/2014/09/displaying-your-ipad-on-your-classr...