public string Proc_test(int student_id, int class_id)
        { 
            string str_conn= string.Format("Data Source=DADI-PC\\MYSQL2008;Initial Catalog=Test;Persist Security Info=True;User ID=sa;Password=mysql2008");
            using (SqlConnection conn = new SqlConnection(str_conn))
            {
                conn.Open();
                SqlParameter par1 = new SqlParameter("@stu_id", SqlDbType.Int);
                par1.Value = student_id;
                SqlParameter par2 = new SqlParameter("@class_id", SqlDbType.Int);
                par2.Value = class_id;
                SqlParameter par3 = new SqlParameter("@nun", SqlDbType.VarChar, 50);
                par3.Direction = ParameterDirection.ReturnValue;

                SqlCommand cmd = new SqlCommand("proc_test", conn);
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.Add(par1);
                cmd.Parameters.Add(par2);
                cmd.Parameters.Add(par3);
                cmd.ExecuteNonQuery();
                return cmd.Parameters["@nun"].Value.ToString();
            }

        }

ALTER proc [dbo].[proc_test] 
@stu_id int,
@class_id int,
@nun varchar(50) output
as 
begin
    if exists( select student_str_id from Student where Student_id=@stu_id and Class_id=@class_id)
    select @nun='true';
    else
    select @nun='flase';
end