When change StudentID TO identity not work why

Question

I have Table student created in sql server 2005 by the class Student as following

 

namespace UniversityData.Models
{
    [Table("Student")]
    public partial class Student
    {
        public Student()
        {
            this.Classes = new HashSet();
        }
        [Key,DatabaseGenerated(DatabaseGeneratedOption.None)]

        public int StudentID { get; set; }

        [Required]
        [StringLength(50)]
        public String StudentName { get; set; }
        [ForeignKey("Section")]
        public int SecID { get; set; }
        public ICollection Classes { get; set; }
        public virtual Section Section { get; set; }


    }
}

I have already Table Student in database with no column identity

I try to change StudentID to identity

so that  i add DatabaseGeneratedOption.identity in place of DatabaseGeneratedOption.None

and i add in container class the following

 

protected override void OnModelCreating(DbModelBuilder modelBuilder)
       {

           modelBuilder.Entity()
    .Property(c => c.StudentID)
    .HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);

 After that write in console add-migration aaa

it generated migration as following

 

 

public partial class aaa : DbMigration
   {
       public override void Up()
       {
           DropForeignKey("dbo.Student_Class", "StudentID", "dbo.Student");
           DropPrimaryKey("dbo.Student");
           AlterColumn("dbo.Student", "StudentID", c => c.Int(nullable: false, identity: true));
           AddPrimaryKey("dbo.Student", "StudentID");
           AddForeignKey("dbo.Student_Class", "StudentID", "dbo.Student", "StudentID", cascadeDelete: true);
       }

       public override void Down()
       {
           DropForeignKey("dbo.Student_Class", "StudentID", "dbo.Student");
           DropPrimaryKey("dbo.Student");
           AlterColumn("dbo.Student", "StudentID", c => c.Int(nullable: false));
           AddPrimaryKey("dbo.Student", "StudentID");
           AddForeignKey("dbo.Student_Class", "StudentID", "dbo.Student", "StudentID", cascadeDelete: true);
       }
   }
But finally not working what identity not working
Please help me in that if possible
I attached my topic with message