When my students go to assignments to complete a quiz or test they won't load either. Is there a setting that I need to change in my classes? I had them set up with video & article assignments, do I need to make sure that I don't assign them so that they can access them without watching a video or looking at an article. 

If you're not getting the answers you need here quickly enough, I suggest submitting a support request directly to Khan Academy staff by using "Report a problem" in the upper right corner.