there is a possibility, incase your country system is as outdated as is mine then the grades are saved on school's main computer, then after some (unknown) time they are posted on ministry of education database, so.. if its that your case you just need to inject a trojan on that computer and do whatever you want there from your pc or just sneak into the office when nobody is there and do the changes as first person, if grades are set by teachers personally into the education ministry platform then you either have to hack the teachers' accounts or you have to hack the ministry itself and give yourself a post graduate degree for free