When you triple-click the home button, as Garethmaybury said, remember to hit "end" in the top left of the screen.
if you get guided access to stop, a faster way to turn it off so as to prevent the mistake of entering it again (if that's what you want to do.) You can open Siri and simply say "turn off guided access."
unfortunataly Siri is not usable while guided access is in use (meaning, after you triple hit the home button and tap start in the top right,) when you try, the top of the screen notifys you to triple hit the home button to end guided access.
I Played with guided access for a bit and found that and this, you can't use guided access while in the system setting, I don't know why, or it may just be my iPad, but it doesn't work.
hope you find the answer soon,
Furfox.