Use the default clock on your iPad, paired with a passcode, to turn the iPad (or iPhone) off automatically. Follow this easy 2-step process to limit the time a student has in an app:
Open the Clock app, and choose the Timer icon at the bottom of the screen. Tap the music icon at the bottom of the timer screen (♫ ♫) and choose “Stop Playing”, then “Set”. Open the Settings app, tap Passcode in the left column, then Turn Passcode On in the right column, and set a 4 digit passcode (DO NOT FORGET THIS PASSCODE!) Make sure “Erase Data” slider is off on this screen.
To use this feature, return to the clock app, Set Timer > Start. Give the iPad to the student with the desired app open. When the timer runs out, the Slide to Unlock screen will be shown and the passcode must be entered to unlock. (Click here for printable directions with pictures).