SQL Server 2017

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CREATE TABLE StudentTable
(
    [StudID] INT
   ,[Name] VARCHAR(12)
   ,[Class] VARCHAR(12)
   ,[Marks] TINYINT
);


      
      
INSERT INTO StudentTable ([StudID], [Name], [Class], [Marks])
VALUES ('2003', 'aman', 'X-A', '91')
      ,('2005', 'ankita', 'X-A', '89')
      ,('2010', 'Aakash', 'X-A', '87')
      ,('2011', 'Cyril', 'X-A', '87')
      ,('2012', 'Bala', 'X-B', '87')
      ,('2013', 'Sara', 'X-C', '89')
      ,('2014', 'Katlyn', 'X-C', '89')
      ,('2015', 'Casy', 'X-C', '87')
      ,('2016', 'Katie', 'X-B', '93');
9 rows affected

SELECT *, 
(
 SELECT COUNT(DISTINCT s2.Marks) 
 FROM StudentTable s2 
 WHERE s2.Class = s.Class 
   AND s2.Marks >= s.Marks
) AS Rank
FROM StudentTable s
ORDER BY Class, Marks DESC;


StudID
Name
Class
Marks
Rank


2003
aman
X-A
91
1

2005
ankita
X-A
89
2

2010
Aakash
X-A
87
3

2011
Cyril
X-A
87
3

2016
Katie
X-B
93
1

2012
Bala
X-B
87
2

2013
Sara
X-C
89
1

2014
Katlyn
X-C
89
1

2015
Casy
X-C
87
2

StudID	Name	Class	Marks	Rank
2003	aman	X-A	91	1
2005	ankita	X-A	89	2
2010	Aakash	X-A	87	3
2011	Cyril	X-A	87	3
2016	Katie	X-B	93	1
2012	Bala	X-B	87	2
2013	Sara	X-C	89	1
2014	Katlyn	X-C	89	1
2015	Casy	X-C	87	2