S Rank

Every action in Yandere Simulator can result in an "innocent point" or a "guilty point."

If the police are called to school, that's +1 guilty point.
If the police discover a dead body, that's +1 guilty point.
If you alarm a student, that's +1 guilty point.
If a student sees you covered in blood, that's +1 guilty point.
If a student sees you carrying a weapon, that's +1 guilty point.
If a student testifies that they witnessed you commit murder, that's +20 guilty points.

If a girl vanishes, gets arrested, gets expelled, or if her death is judged to be an accident, you do not gain any guilty points.
If a girl's death is judged to be a murder, that's +10 guilty points.

Making a friend is +1 innocent point.
A single point of reputation is +1 innocent point.
Convincing a girl to testify that you're a good person is +10 innocent points.

Your rank at the end of the game is determined by the number of innocent points you have, versus the number of guilty points you have.

If you have more guilty points than innocent points, you get an "F".
If you have at least 100 more innocent points than guilty points, you get an "S".
Any number in-between will get you a grade somewhere between "D" and "A".

Theoretically, you could have 100 guilty points and still get an S rank, as long as you have 200 innocent points.

S+ Rank

Between each week of gameplay, the player is presented with a cutscene where the protagonist describes the most fitting method to eliminate her next rival. If you meet the criteria to obtain an S Rank, and you also eliminate each rival the way that Ryoba describes, you will achieve the "S+ Rank", a special rank that unlocks an extra cutscene after the credits finish rolling.