As noted by @cjdelbalso , there is also an option for disabling a specific area of the screen:
"One feature that is not shown in the video is the function to disable specific areas on the screen. When you are in the main guided access menu, simply use your finger to draw a box or circle around the area you want to disable. This feature is very helpful if your kids are using an app that has ads or buttons you don’t want them to click on."
This feature is highly recommended for younger students whenever they use your class iPads.