Hiding the address book on a single multidomain tenancy 0365 setup


I look after a trust called NET and now more schools have joined this trust. Each of the school running different email system. I have been asked by the NET management to design one 0365 email system for the whole trust. So I have subscribed for office 365 and have set up a single multi domain tenancy for the trust.
At the moment there are 15 schools within the trust and in the future more schools may join our trust.

I did some research and Microsoft article says that I need to create a single Azure AD connector and sync all the 15 active directory into this single Azure AD connector and this will create mail accounts in the 0365 and this will have a one GAL.
I am bit concerned in the future if more schools joins the trust and the address book is going to be very huge.

Is there any way address book can be hidden from each other viewing-means when one of the school (Domain-A) after migrating to the 0365 single multi domain tenancy(NET).
When the Staff from Domain-A log into 0365, I would like them to see only the staff from Domain-A + Students from Domain-A and All the staff from Domain-B, C, D, E, F…..O

Similarly when Students from Domain-A log into 0356, I would like them to see only the staff from Domain-A + Students from Domain-A only.

Please let me know if this is possible to achieve or if there is a better way of planning and setting up this.
Any suggestions much appreciated and thanks in advance.
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Cliff GaliherCommented:
Realistically this should be multiple tenants for their individual domains. If the trust wants their own domain for organization-level communications  that'd be another tenant. But having each school with a unique foreat/domain be all one big tenant is a recipe for problems down the road.

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Vasil Michev (MVP)Commented:
If you are "dumping" those in the same O365 tenant, the only solution will be to use the GAL segmentation feature to create separate ALs/GALs per organization: https://technet.microsoft.com/en-us/library/jj657455(v=exchg.150).aspx
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