One question I get from many parents and teachers (and other professionals), is how to control the amount of time my child can use the iPad. Some kids have a hard time with timers that do not actually end their time on an iPad (such as a timer set on a table). I came across this video showing how to set the timer on the iPad (or iPhone) and actually STOP what is going on on the screen. When the timer ends, the iPad will return to the lock screen, and when a passcode is set up, the child cannot get back into the device without the code.
The video creator, Jim Kauppila, demonstrates using the music application. I tried the same technique using Angry Birds, with the same result. It appears that it will return to the lock screen, no matter which app is being used at the time.
This is a great option for parents and teachers to help regulate iPad time in an easy way! Watch the video, and see if this is a method that will work well for your circumstances.