Guided Access problem iOs 9: iPad doesn't sleep

In the past, iPads in Guided Access time out after some inactivity, saving battery.

After updating most of our iPad (all kinds) to iOs 9 and iOs 9.0.2 the iPads stay on in Guided Access (the screen dims) until someone manually turns it off with the enabled sleep/wake button. The point of Guided Access was to increase independence of students. An adult wouldn't have to check if it was turned on or off at the end of class.

This is a problem because most students don't manually turn off the iPads, and they run out of battery before making it through the school day.

Is this a bug? Please tell me there will be a fix...