Back button not working intuitively
The navigation between students is not intuitive: The "previous" button is not the inverse of the "next" button. Also only ever pressing "next ungraded" and then pressing "next" will sometimes result in a student that has been graded already (you would expect this to only happen if you loop back to the first student after seeing everyone, this is not the case).
To the user it does not seem like there is a fixed order of students between which you navigate. This is problematic as it does not allow you to quickly review the student you graded moments ago.