Access modifier : "private" for distinct instances of the same class

Many of us who are reading this blog might have learned the use of access modifiers in Java. I found that there is a small confusion about the use of "private" access modifier.

private	member	Accessible only in its class(which defins it).
protected	member	Accessible only within its package and its subclasses
public	class interface member	Accessible anywhere Accessible anywhere Accessible anywhere its class is.

(Taken from http://www.javacamp.org/javaI/modifier.html)

Let us create a new class named "Student".

public class Student {
 private int score;

 public Student() {
     score = 89;
 }

 public void modifyScore(Student that,int newValue){
     that.score = newValue;
 }

 public String toString() {
     return "Score = " + score;
 }

}

Imagine a situation where instance "A" of class "Student" trying to modify the "score" member of instance "B" of the same class, The misunderstanding is this cannot be done because of the score's access modifier is private. In fact, it can be done correctly. As the rule states that private members are accessible only in its class. So instances could access the private members of other instances too.