How can I force my iPad to lock? There are certain times when I would like to be able to force my iPad to go to passcode lock mode immediately, rather than waiting for the usual 15-minute timeout. 

I currently have "require passcode after" set to 15 minutes. There is also an 'immediately' setting, which requires a passcode every time that the device is brought out of sleep mode, no matter how much time the device was asleep. 

I am wondering if there is a way to put the iPad into 'require passcode immediately' mode on a one-time basis, without changing the 15-minute default that I have in settings. The instances where I need an "immediate lock" are few and don't outweigh the utility of keeping the 15 minute delay most of the time.

Is this possible? If so, how?
Turning the iOS device off will require a passcode to be entered no matter the timeout value if you have one other than requiring the passcode immediately upon locking.

You can also utility any of the "Find my iPhone" or MDM tools that allow remote locking of the device, but these are often not feasible and much more time and work than simply powering off the device or rebooting it.