iPad stuck in Guided Access
If you continue to be unable to guess the passcode by brute force attempts, you can connect the device to a computer that has synced the device and unlock things with a sync.

Also, this article shows an alternate method involving a reboot, force quit, and other manipulation of settings in a specified order. Hopefully it helps if you can't use the two "approved" methods above.

http://blog.technguide.net/2012/08/how-to-get-out-of-guided-access-on-ios6/


  
  Hold the Lock Button
  Slide the Slide To Power Off slider [If touch is disabled/you're unable to slide it, do a hard reset — hold the lock and home button for 8 seconds until the Apple Logo appears]
  When your iDevices powers up, immediately exit the app that automatic open.
  Go to Settings > General > Accessibility > Guided Access
  Click Set Passcode
  Enter new password
  Reboot again.
  Wait for the app to automatically open and Triple tap the home button
  Enter the new password
  Unlocked!