Generate model-based migration (Entity Framework)

I'm trying to generate a migration based on a model:

  [Key]
    public int Id { get; set; }
    [MaxLength(100)]
    public string NomeInstituicao_Fundamental { get; set; }
    public string AnoInicio_Fundamental { get; set; }
    public string AnoFim_Fundamental { get; set; }
    [MaxLength(100)]
    public string NomeInstituicao_Superior { get; set; }
    public string AnoInicio_Superior { get; set; }
    public string AnoFim_Superior { get; set; }

    public Aluno aluno{ get; set; }

But after running the command: Add-Migration , my migration is generated, but the Up () function is empty. I need it to already be modeled, just like the attributes defined in my model.

 public partial class AcademicosMig : DbMigration
{
    public override void Up()
    {

    }

    public override void Down()
    {
    }
}

Context:

    public class Context : DbContext
{
    public Context() : base("name=Default")
    {
        this.Configuration.LazyLoadingEnabled = false;
        this.Configuration.ProxyCreationEnabled = false;
    }
    public  DbSet<Academicos> Academico_ { get; set; }
}