I think replacing the grade is sufficient. The grade is a demonstration that the student has mastered the content covered on the assessment. The original grade no longer reflects what the student knows. Another option is to average the two scores, and replace the average with the original score.

I would think it could be dependent upon the reason why Johnny is re-taking the assessment. If I had a student perform poorly when they were sick, dealing with a death in the family, or some extreme situation I would replace the grade with the highest earned (original or re-take grade). If Johnny performed poorly, because he didn't want to make an effort I likely wouldn't allow a re-take in the first place. Instead I would offer Johnny and any other student wishing to earn extra credit the opportunity to do so.

I would think Johnny's teacher's school system has a grading policy. My school's policy regarding grades is as follows: if a student fails an assessment, I have to reteach the material and assess the student again. The first grade has to go in my grade book along with the second grade. I also have to change all grades below a 50 to a 50 at the end of the nine weeks before report cards are printed. I feel like the second/re-take grade should go in the grade book if it is a lot better than the first grade/assessment.

I feel that the grade can be replaced but not full credit.

In my opinion, I think Johnny's grade should be replaced for half credit. For example, Johnny could gain a maximum of 30 points when he retakes the assessment.