Are the grades the student received permissible for him, after he postponed the exam with a forged medical report?

The medical report that falsely establishes your illness is a false testimony, and one of the greatest major sins that the Prophet, peace and blessings be upon him, forbade. Therefore, you and the doctor must repent and seek forgiveness, for you have both combined the sin of false speech and lying, and false testimony, and the sin of cooperating in sin and aggression. As for the grades you obtained from the make-up exam, they are permissible if they were through your own effort and without cheating.