Authenticate different user via portal instead of override?

I've read the forums and can't find a similar request. https://forum.fortinet.com/tm.aspx?m=133794 comes close but there are no responses there.

All our devices get authenticated - even if it just drops to SSO_Guest_users as "guest". We don't currently have any captive portal in place, and I don't necessarily want to stop all internet access until they authenticate. I'm looking more for how to resolve this issue:

We have admin, staff, and student/guest profiles (the filtering goes from minimal to restrictive based on which profile you get). All our iPads get student/guest profile. On the iPad is there a way to get to the FG authentication portal via a URL - so that the user can be deauthenticated from the "guest" account and then authenticated using the new username/password they enter on that webpage (either a staff or admin user)?

I looked at doing the Allow Blocked Override under the student/guest profile, and I may be able to get it to work, but it doesn't get me to the deauthenticate option. 

How can I enable/test the authentication portal? Is that even possible?

TIA,

Michelle