I've discovered that our school can bypass the Microsoft login screen with a redirect, bit of an explanation on this site.
Basically, our students are currently directed to the Microsoft login screen for Office 365 where they have to enter their entire school email address (slightly challenging for our Primary School lower years) before being directed to our school portal login screen. With this workaround, I can get them to go directly to our school portal login screen. I've been able to update the school portal web clips for our iPads with no issues, but for the Macs, trying to update the Plist I use for Safari home page is resulting in an error:
Problem uploading Safari_copy.plist. Plist files must be in XML format. Try '/usr/bin/plutil -convert xml1 /path/to/file.plist'
I changed the url to the below which allowed the plist to be uploaded to the config profile.
When deploying the config profile to my Mac, it fails with the following error:
Unable to decrypt encrypted profile