We have found the cause of the GAL problem. We were using an anonymous bind to ldap to retrieve email addresses and had restricted the number of records returned for anonymous requests. Unlike Zimbra, apparently Outlook requests the entire ldap directory when it starts up. It was coincidence that they were all students (having to do with the order of record entry most likely).
So, on to the next issue. We have assigned everyone "canonical" email addresses in Zimbra in addition to the email account names. We only include the canonical email addresses in the GAL. When I try to share a Calendar (right click on calendar name, Choose properties/sharing/add, and select an address for To

, Outlook does not fill in the selected name as the recipient so I could select what permissions to give. I can repeat the "add" cycle indefinitely, but no recipients are added so I can give them permission to the calendar.
My guess is that Outlook wants email account names in the GAL in order to enable sharing. Is this true? Do we need the email account names stored in our external ldap directory in a particular location?
Richard