So I have a story/guide which will probably help some of you gain access, or "hack" a school computer.
If you think you have to type a command on CMD or install a keylogger, you're wrong. That's probably the dumbest/hardest way to do so, since most school networks/computers have security software which restricts such actions.

Instead, use some social engineering skills.
Become friends with someone who is a TA (teacher's assistant), and get a staff member's password, or stand near them when they type their password(s) and hope you catch it.
Once you get a staff password and username, log in through a school computer when the staff member is not on their computer, otherwise it'll log them off from their computer and they'll suspect something (logging in through a school computer on the same router/connection as the staff member will definitely lower suspicion). I say this because my school had different routers which belonged to certain classrooms/school sections.
Once you log on to whatever account of theirs you want/can, you can just check their records/files and maybe edit some. Example : if your school/teacher uses an online system to input grades for you to see, you can add points towards yours. Try to add points to pre-existing assignments, NOT future assignments, or else it will be noticeable.

When I was in high school (2 years ago), I did something similar to this. I managed to get a bunch of classmates' usernames to our computers. I was tempted to check and see if the computers stored our usernames in a folder, and sure enough, I found a folder with a lot of them. There was this folder in the Program Files named "LogIn" or something like that, and apparently, for some reason, it saved our usernames on it. Getting/guessing some their passwords was easier because our school had a system where your standard password was your student id, and most people did not change the standard password. All I had to do was simply ask for their student ID (SID) by asking them if I could use it to get lunch, since my school had a system where you had to punch in your student ID in order to get your lunch (in order to prevent the same student from getting lunch more than once.) After I got their SID, I'd simply check the username file and try to log in to their account through a school computer.
There wasn't really much to do with this other than change their backgrounds/passwords and watch them think they got "hacked" lol.
I also acquired one of my teacher's Username+Pass by standing next to her when she typed it one day. My first thought was to try and log in to our school website, where most teachers posted grades and graded assignments. I did not know whether or not the username and password would be the same, but they were. After I signed in to her account during that class period one day, I just added points to my existing assignments periodically, and that bumped my grade up quite a bit. I ended up getting an easy A. I DID do my work in that class, but I would've probably got a B- without altering my grade(s).

Anyway, what I'm trying to say is that social engineering can be more useful, and easier, than "hacking" something.