If a mistake was made when grading exams, does the prof have to fix it for every student? I took a university course where the final exam was a multiple-choice type with the answers going into a "scantron" sheet that gets fed into a machine. This is reasonable, since there were about 600 students taking it.

My final grade for the exam was such that I was about 1% short of the next letter grade for the course overall. After all the marks were posted, the exams were made available for pickup to students. I picked mine up and found two questions that I was sure were right but were marked wrong, so I went to see my professor to review the exam and ask about them.

He gave me two answer keys to check - one was the master scantron that was fed into the machine to tell it the correct answers. The other was the master paper exam with the questions (same thing the students got on the exam), where the correct answer was circled. I checked the two questions I was interested in and sure enough the paper exam agreed with my answers, but they were marked incorrectly on the master scantron that was used to grade every student. In other words, whoever filled out the answer key scantron made an error on at least 2 questions and filled in the wrong bubble.

I showed the prof the error and he seemed completely unfazed and not surprised and simply said that yes, he agrees that I picked the right answer and since this actually bumps me up a letter grade he'll fill out the form for the university to adjust my already-posted course grade. He then simply put away the two answer keys and bid me good day.

While I have no concrete proof, I feel like he already knew about the errors and me finding them was not a surprise. However, it seems that he does not intend to regrade all 600 students and is only dealing with the ones that actually come in and spot the error. Is this an acceptable approach? So far he has sent no communication out to the class regarding this and to my knowledge most of the students are unaware of the error.
For me, all the papers should be re-graded with the corrected marking scheme - ethically correct and supported by any future checking.

Edit: And as per Fabio’s comment - have had to do similar corrections due to my error or poor question so it happens but not too often...