Answer:-

 Thanks all for answering that question. I really appreciate the effort of all the people who tried and gives the response.We get multiple answers for this question in which correct answer is given below:-

Correct answer in case the first coulmn is an identity column

Answer given by Arjan  Dingemans


WITH Numbers AS (
SELECT 1 AS Number
UNION ALL
SELECT Number + 1
FROM Numbers
WHERE Number < IDENT_CURRENT('tbl_students')
)
SELECT Number
FROM Numbers N
LEFT JOIN tbl_students S ON N.Number = S.StudentId
WHERE S.StudentId IS NULL

Answer given By Chandra Prakash Singh

select number from master..spt_values s
left join tbl_students on s.number = tbl_students.studentid
where s.type='P'
and s.number <= IDENT_CURRENT('tbl_students')
and tbl_students.studentid is null and number > 0

Correct answer in case the first coulmn is not an identity column 

Answer given By Avinash Dubey

WITH DeletedRows (missnum, maxid)
AS
( 
SELECT 1 AS missnum, (select max(Studentid) from tbl_Students) UNION ALL SELECT missnum + 1, maxid FROM DeletedRows 
WHERE missnum < maxid
)
SELECT missnum FROM DeletedRows
LEFT OUTER JOIN tbl_Students e on e.Studentid = DeletedRows.missnum
WHERE e.Studentid is NULL OPTION (MAXRECURSION 0)

Answer given by Aviral Lamba

select number from master..spt_values s
left join tbl_students on s.number = tbl_students.studentid
where s.type='P'
and s.number < (select MAX(studentid) from tbl_students)
and tbl_students.studentid is null and number > 0 

Also I appreciate the effort done by chetan Deshpande in attempting the question