Wow, that'd be a lot of contacts in a large organisation.
Populating the user's My Contacts would be synced to the user's phone (which is the point if I'm reading this right).
Could potentially "blow up" someone's phone. :-)
Well, I still do this manually on every incoming call, adding them as contacts, which perhaps I wouldn't have to if I did this.
Feels like I'd like to skip all students, though.
And, I'd still not get phone numbers of desk phones in classrooms, and staff areas, as they are not users in the GAL.
For private callers I use the Truecaller mobile app, that does automatic lookups during the incoming call. Also lets me auto-block spammers/telemarketers.