Logout/deauthenticate via URL? Captive Portal

Is there a URL you can load that will deauthenticate the captive portal session?

The situation is in a school, we have a few shared sets of iPads. For safeguarding we have to know who has accessed what so we set the iPads up with captive portal authenticating against our Windows AD. That works fine. The issue is they remain logged in until the set timeout interval. So if another pupil uses a shared iPad before it times out all activity is tied to the previous persons username. 

With the iPads we can push home screen icons that point to a website, so is there a URL i can load that will tell the fortigate to deauthenticate the device it receives the request from? That way i can create a logout icon on the iPads.