How to retrieve data of nested class instance from parent class instance in ASP.NET Core?

I am trying to modeling a class at school, and I end up with something like this:

public class Class
{
    public int ID { get; set; }
    public int Grade { get; set; }
    public Teacher ClassTeacher { get; set; }
}

This is the Teacher class:

public class Teacher
{
    public int ID { get; set; }

    [Display(Name = "First Name")]
    public string FirstName { get; set; }

    [Display(Name = "Last Name")]
    public string LastName { get; set; }

    [DataType(DataType.Date)]
    public DateTime Birthday { get; set; }
}

When I use scaffolding, migrate and update the database, this is the structure Entity Framework built for me:

dbo.Class:
ID: int
ClassTeacherID: int
Grade: int

dbo.Teacher:
ID: int
Birthday: datetime2(7)
FirstName: nvarchar(MAX)
LastName: nvarchar(MAX)

I want to display the Teacher's FirstName in Views\Classes\Details.cshtml, but the Model.ClassTeacher is null, even after I created a Teacher instance in the database and set ClassTeacherID to the newly created Teacher's ID. Looking for your helps.