Try this
select Cast(count(*) as char(10)) as StDate
from tblStudents where studentNumber = 12213123;
You can also use the CASE statement, and if the Student Number is unique, the max() or min() functions to get the stDate from the table.
select Cast(count(*) as char(10)) as NumRows,max(stDate) as stDate
from tblStudents where studentNumber = 12213123;
If you only want a single field, try something like this
select
CASE count(*)
WHEN 0 THEN ''
ELSE CAST(max(stDate) as char(12))
END CASE as StartDate
from tblStudents where studentNumber = 12213123;