iPads and iPhones have a tool called Guided Access that allows you to restrict a user to a single application. This is often desired when you allow young fingers to use your phone, but works well in the classroom, too. A passcode is needed then to turn off Guided Access.
(Teachers: It's best not to give out this code since students could then change it)
Here's how it works:
Below is a screenshot from iOS 9.3 in the Settings, showing where to turn on Guided Access:
Once it is turned on, go to Passcode Settings
Set a 4-digit code that will let you 'unlock' an application.
This can be different than the passcode you may have set to get into the iPad.
Navigate to and open the application you would like to lock.
'Triple-click' the home button.
Several options appear at the bottom of the screen to select from
Hardware Buttons: toggle on or off your sleep or or volume buttons
Time Limit: access time to the application can be limited here
In the upper-right corner of the screen, select Start (or Resume)
To 'unlock' the application
'Triple-click' the home button
Enter your passcode
Tap End in the upper right corner of the screen
