To setup guided access go to Settings, General, Accessibility, and then Guide Access. Click on the toggle to turn it on, enter passcode settings, and finally toggle the accessibility shortcut. This allows you to simply triple click to start and end a guided access session. This can be a minor irritation later when students accidentally triple click but it is a quick pin entry to end guided access.
To start a guided access session simply open the app you want students to use, triple click the home button and choose start. You might want to click on the Hardware Buttons - Options button to allow use of the sleep button and volume buttons so that kids can put the iPads to sleep and control the volume if they are watching a video. Students will not be able to leave that app without the pin entered. Guided access does not stop students from going to different websites if they are in Safari, it only locks them into the app it is enabled with. If you triple click with just the home screen showing nothing will happen. You have to go into the app first and then triple click. Hope this helps. Happy Teaching.