#1 I have done it #2 I was smart enough to not Change grades #3 if you are dumb enough to try to change grades have fun in jail..
Now if you still want to continue after what I said you need to get the version of their computer that they are using Mac Sierra 10.2 windows 10 etc once you have that you need to look for exploits into the system when you are done with that you need to basically start infecting the school, if you are noticed you will be fucked so I highly recommend using multiple layers of proxies etc ( this all goes on fact that school provides you with PCs) main point of this is you need to work your way up the security ladder, students can look at their grades teachers can change them and others can do a lot more. Pint is get into the lowest level of the system and start infecting your way up wether it be via rat, social engineering etc. I hacked my school in 9th grade in under a minute it was very easy but it’s not going to be very easy for you FYI I’m 17