Can't suggest a full solution without seeing your code and database structure but, you may follow these steps to get it solved-
1. Set
AutoPostBack property of dropdownlist to
True
2. In
SelectedIndexChanged event of the dropdownlist, retieve required data from database. Query something like following may help
SELECT ColName
FROM TableName
WHERE StudId=@StudId
AND DATEPART(MONTH,AttendanceDate)=@Month
3. Save the result to a string variable
4. Assign the value kept in the string variable to the text.
Hope, it helps :)