Woohoo, Since I got upgraded on PrimaryEmail I had access to this new calendar sharing thing. Currently I’m sharing my free/busy but you can share anything you want. It works really well!
To enable sharing your calendar:
Log in to your Primary Email or School Email account via the website.
Click Publish this calendar to the internet
Change your settings and then get the Link for viewing calendar in a Web browser
If you want to embed your calendar copy the Link for viewing calendar in a Web browser URL and embed it in an iframe, it will look something like this:
<iframe src="http://exchange.schoolemail.co.uk/owa/calendar/John@primaryt.co.uk/Calendar/calendar.html" width=600 height=600></iframe>