Select *, row_number() Over(Partition by StudentID Order by EntryDate)
 From Students

DECLARE @students TABLE (student VARCHAR(20), attendDate DATETIME)

INSERT @students 
	SELECT 'student1','1/1/2008' UNION
	SELECT 'student1','2/1/2008' UNION
	SELECT 'student1','3/1/2008' UNION
	SELECT 'student2','2/1/2008' UNION
	SELECT 'student2','3/1/2008' UNION
	SELECT 'student2','4/1/2008' UNION
	SELECT 'student2','5/1/2008' UNION
	SELECT 'student3','3/1/2008' UNION
	SELECT 'student3','5/1/2008'
	
SELECT
    (SELECT COUNT(*) FROM @students WHERE student = s.student AND attendDate < s.attendDate) + 1 AS row
    ,student
	,attendDate 
FROM 
    @students AS s