Urgent help needed here for a school Project

With no great confidence, I'd suggest changing this statement:

SELECT @GPA = CONVERT(decimal(4,2), SUM(@CreditHours * CAST(CASE r.Grades WHEN 'A' THEN 5 WHEN 'B' THEN 4 WHEN 'C' THEN 3 WHEN 'D' THEN 2

    WHEN 'E' THEN 1 ELSE 0 END AS int )) / SUM(@CreditHours)) 

FROM [Result]r

WHERE

r.StudentId = @StudentId 

to this:

SELECT @GPA = CONVERT(decimal(4,2), SUM(@CreditHours * (CASE r.Grades WHEN 'A' THEN 5 WHEN 'B' THEN 4 WHEN 'C' THEN 3 WHEN 'D' THEN 2

    WHEN 'E' THEN 1 ELSE 0 END)) / CAST(SUM(@CreditHours) AS decimal)) 

FROM [Result]r

WHERE

r.StudentId = @StudentId 

The cast to decimal on the second operand is to prevent the division been done using integer arithmetic which will truncate the fractional part. The cast to int on the CASE expression result should be unnecessary as it's already an int.

In that case, I don't see how you can get a negative result unless there's something wrong with the expression.

The expression looks OK to me but it is complex. I'd try breaking it up by assigning the CASE expression to a temporary variable and see if that makes any difference to the result. If nothing else, you might be able to figure out more easily where the problem lies.