Go to Settings from your iPad homescreen.
Within Accessibility, find Guided Access under Learning and turn it to ON.
Set a passcode.
Pick a four-digit code that's easy for you to remember, but that your students don't know.
Go back (arrow at top left) and make sure Guided Access is selected for Triple-click Home.
You have enabled Guided Access. Open your app, and triple-click Home. A new screen will pop up, and you will have to hit Start (upper right). A student can't End or switch apps without the passcode.