On the assignment grader screen parts of the table are hidden by the right column (that contains the add a block...block). If the screen resolution or size does not show the grade column in the table due to the right block column then there is no way to get to the grade button for the assignments.
I have tested this in Firefox, Chrome, and Safari on a Mac and with multiple themes.
Other than quick grading or ensuring that the teacher has a large enough display to see the full screen I do not see any other way to grade assignments.
Am I missing something?
Seems like it is related to this bug: MDL-22686 that I reported a couple of days ago.