Just a thought - may be the student_id is the question itself? As in all students got different, unique questions, and therefore the prof can easily identify the person by looking at the question?
I'm guessing you've never had to create a final exam. The mere thought of creating unique questions for each student is enough to make me hibernate all winter.
I had an engineering professor who programmatically created unique exams (random numbers + randomly shuffled types of questions). It actually made his job a lot easier because grading was done via scantron. What I didn't like is there was no partial credit from showing your work.
I've had exams where the question was essentially the same, except that everyone had unique parameters.