Hi mlross,
By my test, office 365 client could connect to on-premise server, include Skype for business client to connect to Skype for business server 2015, you could refer to the following link:
https://technet.microsoft.com/en-us/library/gg702619.aspx
“You can use Office 365 ProPlus with supported versions of Exchange Server or SharePoint Server that are installed on-premises in your organization”
I also do a test for Skype for business server 2015, Skype for business client in Office 365 ProPlus could connect to on-premise Skype for business server 2015.
Best Regards,
Jim Xu
TechNet Community Support
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