When you're ready to quit your app, triple-press the Home button again to re-enter the Guided Access setup screen. You'll be prompted for your passcode first. Then, you can tap End and then the Home button again. Or, to save a little movement, you can to the triple-press of the Home button, then press the Home button again from the Guided Access setup screen, and then a fifth time to exit the app.
Guided Access isn't any more of an ideal solution than the hardware mute switch: It locks out buttons, requires a passcode, and makes you issue a lot more taps when you're ready to move on from the app you were using. But, as I said at the outset, though, both options are better than nothing. And we can keep hoping Apple will implement a real solution sometime soon.
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