Unfortunately public folders are currently only available in the Dedicated version of Exchange Online. MS resources have previously mentioned in these forums that you can file a feature request by clicking the support tab through the Admin Center (
https://admin.microsoftonline.com ) and filing a service request with support indicating you would like public folders to be available in the standard Exchange Online offering.
Users can share their calendars with each other through Outlook, or administrators in your MSO environment can create Conference Rooms through the Admin Center to provide general calendars for your office's physical resources. You might check out the Conference Room feature, it might be that you could use that as a shared calendar of sorts if that's not exactly what you had in mind.
John
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