We are an academy with a an on-premises domain setup with an Office 365 tenant using Azure AD Connect, we have no Exchange servers. We have two categories of on-premises domain users, Staff and Students. Staff will be given Exchange Online mailboxes but Students will either not have access to the tenant at all or will not be given a mailbox, we wish for students to continue using their personal email accounts, their Domain Users have their external SMTP address kept in the 'mail' attribute currently.
We want to use these Student Users in our Office 365 tenant in so far that they can be used with Distribution Lists and appear in the Exchange GAL but it seems Users must have a mailbox or we must use Mail Contacts.
How can I use my Student Users, that have external email addresses, with Distribution Lists and have them appear in the GAL without giving them a mailbox and provisioned email address?