S rank
So you want to rank up? Who doesn’t, well if you want to get S rank and you did not start with it…tough titties. There will only be 5 S rank ninja on this site at any time, this is to stop the top heaviness that most sites get. So if you truly want to get an S rank then here is what you have to do, wait until a tribe head goes inactive and have the other clan heads in your tribe vote you into the spot. Stage a coup and assassinate the tribe head and take over the tribe, or naturally replace him if the tribe head dies during a battle. Killing the tribe head will not get you an automatic rank up, I am sorry but it simply will not.
A rank
This one is simple, you must first be an elite ninja(B rank), and get voted by your clan to replace your current clan head. Now keep in mind that the current clan head does not have to die for this to happen. They can resign and keep their rank, and you can replace them and rank up. Really simple stuff.
B rank
For this you must be recognized by your clan head, and be suggested(IC) for a promotion by your clan head to your tribe leader. Your tribe leader will then issue you a test and if you pass, congrats you get a rank up.
C rank
Simple, once your teacher allows you to pass, you can apply for the Ninja exam. At certain dates every tribe will band together and hold a test for all of their trainee ninja to see if they are ready and up to snuff.(During war time each tribe will have their own tests instead of one big one).