Entity Core, relationships

In my project I have two models: student and occurrence.

In the routine of registering / editing a student, it is quiet, however, in the routine of registering an occurrence, I do not know why, the entity saves nothing of the occurrence, but the complete student.

DbSet adds in the right context, but does not save the instance but the student. I am using the Fluent API to create the tables.

Could anyone help me?

OCCURRENCE

    public class Occurrence : Entity
{
    //Utilizado pelo Entity
    public Occurrence()
    {

    }

    public Occurrence(EOccurrenceType occurrenceType, string cause, string description, EOccurrenceStatus occurrenceStatus, DateTime date)
    {
        OccurrenceType = occurrenceType;
        Cause = cause;
        Description = description;
        OccurrenceStatus = occurrenceStatus;
        Date = date;

        AddNotifications(new Contract()
         .Requires()
         //Validações da data
         .IsLowerThan(Date.Date, DateTime.Now.AddDays(1), "Date", OccurrenceMessages.InvalidDate)
         //Validações da causa
         .HasMinLen(Cause, 1, "Cause", string.Format(SharedMessages.MinLength, "Causa", 1))
         .HasMaxLen(Cause, 255, "Cause", string.Format(SharedMessages.MaxLength, "Causa", 255))
         //Validações da descrição
         .HasMinLen(Description, 1, "Description", string.Format(SharedMessages.MinLength, "Descrição", 1))
         .HasMaxLen(Description, 255, "Description", string.Format(SharedMessages.MaxLength, "Descrição", 255))
         );
    }

    public EOccurrenceType OccurrenceType { get; private set; }
    public string Cause { get; private set; }
    public string Description { get; private set; }
    public EOccurrenceStatus OccurrenceStatus { get; private set; }
    public DateTime Date { get; set; }

    // Um aluno para várias ocorrências
    public Guid StudentId { get; private set; }
    public virtual Student Student { get; private set; }
    //relacionamento com usuário

    public void addStudent(Student student)
    {
        Student = student;
    }
}

STUDENT

 public class Student : Entity
{
    private IList<Occurrence> _occurrences;
    private IList<Parent> _parents;

    //Utilizado pelo Entity
    public Student()
    {
        _occurrences = new List<Occurrence>();
        _parents = new List<Parent>();
    }

    public Student(Name name, 
        Address address, 
        DateTime birthDate, 
        ETypeOfEducation eTypeOfEducation, 
        DateTime academicYear, 
        int serie, 
        string grade, 
        EShifts shifts, 
        int calledNumber, 
        string note)
    {
        Name = name;
        _parents = new List<Parent>();
        _occurrences = new List<Occurrence>();
        Address = address;
        BirthDate = birthDate;
        ETypeOfEducation = eTypeOfEducation;
        AcademicYear = academicYear;
        Serie = serie;
        Grade = grade;
        Shifts = shifts;
        CalledNumber = calledNumber;
        Note = note;

        AddNotifications(new Contract()
         .Requires()
         //Validações da serie
         .IsTrue(Serie > 0, "Serie", StudentMessages.InvalidSerie)
         //Validações da turma
         .IsNotNullOrEmpty(Grade, "Grade", StudentMessages.IinvalidGrade)
         .HasMaxLen(Grade, 1, "Grade", string.Format(SharedMessages.MaxLength, "Grade", 1))
         );
    }

    public Name Name { get; private set; }
    public Address Address { get; private set; }
    public DateTime BirthDate { get; private set; }
    //Verificar depois uma forma de implementar
    //public string Photo { get; set; }
    public ETypeOfEducation ETypeOfEducation { get; private set; }
    public DateTime AcademicYear { get; private set; }
    public int Serie { get; private set; }
    public string Grade { get; private set; }
    public EShifts Shifts { get; private set; }
    public int CalledNumber { get; private set; }
    public string Note { get; private set; }

    //Relacionamentos: 1 escola N alunos
    public School School { get; private set; }
    //Varias ocorrencias para um aluno
    public IReadOnlyCollection<Occurrence> Occurrences { get { return _occurrences.ToList(); }}
    // Varios parentes para um aluno
    public IReadOnlyCollection<Parent> Parents { get { return _parents.ToArray(); }}

    public void SetSchool(School school)
    {
        School = school;
    }

    public void AddOccurrences(Occurrence occurrence)
    {
        _occurrences.Add(occurrence);
    }

    public void AddParents(Parent parent)
    {
        _parents.Add(parent);
    }
}