TreeView的层次结构数据模板

好的，我现在不明白。我在谷歌和Stackoverflow上搜索了一段时间，看到了很多如何使用HierarchicalDataTemplate的例子

我有一个名为Class的类，它看起来像：

[Table(Name = "Class")]
public class Class
{
[Column(IsDbGenerated = true, IsPrimaryKey = true)]
public int Id { get; private set; }
[Column]
public string ClassName { get; set; }
}

我有一个名为Pupil的类，它看起来像：

[Table(Name = "Pupil")]
public class Pupil
{
[Column(IsPrimaryKey = true, IsDbGenerated = true)]
public int Id { get; private set; }
public Name Name
{
get { return this.nameEntityRef.Entity; }
set
{
this.nameEntityRef.Entity = value;
this.NameId = value.Id;
}
}
public Address Address
{
get { return this.addressEntityRef.Entity; }
set
{
this.addressEntityRef.Entity = value;
this.AddressId = value.Id;
}
}
public Class Class
{
get { return this.classEntityRef.Entity; }
set
{
this.classEntityRef.Entity = value;
this.ClassId = value.Id;
}
}
private EntityRef<Name> nameEntityRef;
private EntityRef<Address> addressEntityRef;
private EntityRef<Class> classEntityRef;
[Column]
internal int AddressId { get; set; }
[Column]
internal int NameId { get; set; }
[Column]
internal int ClassId { get; set; }
}

我介绍了一个名为ClassPupils的类，它看起来像

public class ClassPupils
{
public ClassPupils(Class @class)
{
this.Class = @class;
this.Pupils = new List<Pupil>();
}
public Class Class { get; set; }
public List<Pupil> Pupils { get; set; }
}

ClassPupils应该容纳一个班的所有学生。

现在我想创建一个TreeView，其中所有的Pupil都列在Class下面。因此，我在ViewModel中使用了ObservableCollection<ClassPupils>。在我看来，我已绑定到此集合。TreeView的我的视图代码看起来像：

<TreeView Grid.Row="0" Grid.Column="0" Margin="2" ItemsSource="{Binding ClassPupils}">
<TreeView.Resources>
<HierarchicalDataTemplate DataType="{x:Type model:ClassPupils}" ItemsSource="{Binding Class}">
<Label Content="{Binding Class.ClassName}"/>
</HierarchicalDataTemplate>
<DataTemplate DataType="{x:Type entity:Pupil}">
<Label Content="{Binding Name.Lastname}"/>
</DataTemplate>
</TreeView.Resources>
</TreeView>

但我只得到班级的TreeViewItems，而没有得到学生的姓氏。我做错了什么？