You enable Guided Access through Settings > General > Accessibility > Guided Access
Once inside Guided Access, turn it on and click the Set Passcode tab. It will prompt you to input the code twice to ensure it is what you want it to be. Hint: Do not make this easy to hack, such as 1234 or 1111. My first graders tried that right off the bat.
Once Guided Access has been enabled, you can decide when you need to use it. Simply enter the app for use, and triple click the Home Button. You will enter a precursor screen that allows you to circle features to be disabled, and select some of the other options I mentioned above. Click Start and the student is locked into that app with that volume level. When the learning session is over, the teacher can disable Guided Access by triple clicking the home button again and inputting the code.
There is a slide show with pictures and annotations embedded below that explains this process.