How can I group student scores into quintile using SQL Server 2008

Question

Can anyone help me to group student scores into quintile? I think there is a feature in SQL Server 2012, but still we havent upgraded to it as we are using 2008R2. I tried

Answer 1

Below is the correct answer given by Erland Sommarskog 
Create Table #Scores(Student varchar(20), Score int); 
Insert #Scores(Student, Score) Values 
('Student1', 20) 
,('Student2', 20) 
,('Student3', 30)
,('Student4', 30)
,('Student4', 30)
,('Student4', 30)
,('Student5', 40)
,('Student6', 40)
,('Student7', 50)
,('Student8', 50)
,('Student9', 60)
,('Student10', 70)
,('Student11', 70) 
,('Student12', 80) 
,('Student13', 80) 
,('Student14', 90); 

; WITH quintiles AS (
    SELECT Score, ntile(5) OVER(ORDER BY Score) AS quintile 
    FROM   (SELECT DISTINCT Score FROM #Scores) AS s 
)
SELECT s.Student, s.Score, q.quintile
FROM   #Scores s
JOIN   quintiles q ON s.Score = q.Score
go
DROP TABLE #Scores

--by Erland Sommarskog``

Answer 2

Borrowed from marc_s +1

DECLARE @Students TABLE (StudentID INT IDENTITY(1,1), StudentName VARCHAR(20), Score INT)

INSERT INTO @Students(StudentName, Score)
VALUES ('Student 1', 20), ('Student 2', 20), 
('Student 3', 30), ('Student 4', 30), 
('Student 5', 40), ('Student 6', 40), 
('Student 7', 50), ('Student 8', 50), 
('Student 9', 60), ('Student 10', 70), 
('Student 11', 70),('Student 12', 80), 
('Student 13', 80),('Student 14', 90)

SELECT s.StudentName, s.Score, qm.maxQ
  FROM @Students as s
  join ( select score, MAX(Quintile) as maxQ
           from ( SELECT Score, Quintile = NTILE(5) OVER(ORDER BY Score)
                    FROM  @Students ) q 
          group by q.score ) qm
    on qm.Score = s.Score

Answer 3

You must have been doing something wrong when using NTILE(5) - that IS the function to use!

Here's my test setup:

DECLARE @Students TABLE (StudentID INT IDENTITY(1,1), StudentName VARCHAR(20), Score INT)

INSERT INTO @Students(StudentName, Score)
VALUES ('Student 1', 20), ('Student 2', 20), 
('Student 3', 30), ('Student 4', 30), 
('Student 5', 40), ('Student 6', 40), 
('Student 7', 50), ('Student 8', 50), 
('Student 9', 60), 
('Student 10', 70), ('Student 11', 70), 
('Student 12', 80), ('Student 13', 80), 
('Student 14', 90)


SELECT 
    StudentName, Score, 
    Quintile = NTILE(5) OVER(ORDER BY Score)
FROM    
    @Students

And the output is: