How do I get out of the queue app and how do I lock it again?

"Guided access" is the name of the mode where you can only reach the queue app, so that unauthorized people can't use the device for other things or change any settings. Here's how to go out and into "Guided access".

 

Use a narrow tool, such as a paperclip, and push three times in quick succession into the hole that is under the iPad display in the center of the white frame. Enter the PIN code and select 'Finish' in the upper left corner. "Guided access ended" will show in the middle of the display.