It is also still the case, at least on the current test server installation (oracle) that a student can post to the group calendar.  now, you could call this bug, or just say that the feature you want now exists!

to see this in action, go to any class on the test server and click on the red plus icon in any view, even the calendar portlet on the class home page.  you'll get the add an item page, with the option to "share" the event with your personal calendar or the class calendar.  voila!

at one point, there was a bug ticket for this, but i couldn't find it in the tracker this am.