Uvic's php.ini sets the config item session.cookie_domain to "uvic.ca". So, if one tries to write values to a session from "canadianmysteries.ca"
(running on uvic's server) and then read them on a subsequent page, the session variables are not available.
Before starting the session, you have to include ini_set('session.cookie_domain', (strpos($_SERVER['HTTP_HOST'],'.') !== false) ? $_SERVER['HTTP_HOST'] : '');
to set that config item to whatever the current host is (canadianmysteries.ca, canadianmysteries.uvic.ca, or whatever)
I've implemented this in the teacher login section of the CanMys site, but have commented out the code with the explicit SID call, just in case we have to go back to it. Thanks to Ryan for identifying the specific solution to the problem.