Adding user mailbox's hybrid exchange

Hi all, I am relatively new to exchange and have inherited a Hybrid Exchange 2013 / Office 365 setup. With staff members on local exchange and students on 365. I have created a several new users in AD but can’t figure out how to add mailbox’s for them. The company that did the original setup left no documentation and I hit a pay wall when I tired asking them for any information. I would really like to know if this can be done via the PowerShell exchange management console or if I could use dir sync to automate the creation in any way depending on OU.
The setup
1 x Local Exchange 2013 server holding all staff mailboxes
1 x ADFS server
1 x ADFS proxy server
Current Domain: School.org.uk
Office 365 Domain: SchoolSite.co.uk
All current staff email extension (and a few on 365): SchoolSite.co.uk
All Student (365) email extensions: Students.schoolSite.co.uk
All new students I wish to add are in a unique OU in AD, when I launch the Exchange ECP I have an enterprise and office 365 option at the top of the page. Under enterprise I can add a new mailbox but not under 365. When I go to add a new mailbox in Enterprise I can see the option for 365 mailbox but I can see a way to link it to an existing user I don’t know what I am missing but it’s all confusing as. Thanks in advance