how to calculate the position of students in each subject ?

You could use the Linq OrderBy method:

class student
    {
    public int marks;
    public student(int mark) { marks = mark; }
    }
void myButton_Click(object sender, EventArgs e)
    {
    List<student> list = new List<student>();
    list.Add(new student(7));
    list.Add(new student(3));
    list.Add(new student(5));
    list.Add(new student(88));
    list.Add(new student(2));
    list.Add(new student(963));
    list.Add(new student(6));
    list = list.OrderBy(s => s.marks).ToList();
    }

void Main()
{
List<student> list = new List<student>();
list.Add(new Student(7,30)); // position of student = 3rd
list.Add(new Student(3,40));// position of student = 6th
list.Add(new Student(5,60));// position of student = 5th
list.Add(new Student(88,70));// position of student = 2nd
list.Add(new Student(2,5));// position of student = 7th
list.Add(new Student(963,90));// position of student = 1st
list.Add(new Student(6,34));// position of student = 4th
list.Add(new Student(963,343));// position of student = 1st
list.Add(new Student(963,343));// position of student = 1st

list.OrderByDescending (c => c.EngPts).ThenByDescending (c => c.MathPts);
//at this point your list should be sorted like so
/*
963 343
963 343
963 90
88 70
7 30
6 34
5 60
3 40
2 5
which seems to match the ranking you are trying to get
*/
}

class Student
{
//public string positionInEnglish;//position to be calculated and placed in this field for each student
//public string positionInMath;//position to be calculated and placed in this field for each student

public int EngPts{get;set;}
public int MathPts{get;set;}

public Student(int engPts,int mathPts)
{
EngPts = engPts;
MathPts = mathPts;
}
}

this is my 2cents.