+----------------------+
         Class
+----------------------+
  Student_Id (FK, null)
+----------------------+

public class Student
{
   public virtual List<Class> Classes { get; }
}

public class Class
{
   public string Name { get; }
}

public class StudentOverride : IAutoMappingOverride<Student>
{
    public void Override(AutoMapping<Student> mapping)
    {
        mapping
           .HasMany(aggregate => aggregate.Prescriptions).Cascade.AllDeleteOrphan();
    }
}