Visual Bug using LongListSelector (grouped) and Checkbox Windows Phone 8

I'm developing an application that has a grouped list, and within each list item there is a checkbox. The problem that is occurring happens when the checkbox of the first item in group 1 is selected, the checkbox of the first item in group 2 is also selected. But this only occurs visually, in no time the check event is called for the 2 checkbox. I would like to know if anyone has already gone through this, and if yes how to do it, thank you.

EDIT: ----

In my xml I have the LongListSelector:

<phone:LongListSelector 
                x:Name="llsAlunosTurmas"
                Background="Transparent"
                ItemTemplate="{StaticResource AddrBookItemTemplate}"
                LayoutMode="List" />

and the date template of the items:

<DataTemplate x:Key="AddrBookItemTemplate" x:Name="dte" >
            <StackPanel VerticalAlignment="Top" 
                    Margin="0,3,0,0" 
                    Background="White"
                    HorizontalAlignment="Stretch"
                    Orientation="Horizontal">
                <StackPanel Orientation="Vertical" 
                        Margin="12,0,0,0"
                        VerticalAlignment="Center">
                    <TextBlock Foreground="{StaticResource CorTxtPagina}" 
                           VerticalAlignment="Center" 
                           Text="{Binding matricula}"
                           Width="385"
                           FontSize="16"
                           TextTrimming="WordEllipsis" 
                           FontStretch="SemiCondensed"/>
                    <TextBlock Foreground="{StaticResource CorTxtPagina}" 
                           Text="{Binding nomAlu}" 
                           Width="385"
                           FontSize="16"
                           HorizontalAlignment="Left"
                           TextTrimming="WordEllipsis" />
                </StackPanel>
                <CheckBox x:Name="cbxAlunos"
                      Background="{StaticResource CorBotoes}" 
                      Click="cbxAlunos_Click"
                      IsChecked="{Binding selecionado, Mode=OneWay}"
                      Tag="{Binding idtAlu}"/>
            </StackPanel>
    </DataTemplate>

and in C #, after receiving the data by webservice, I organize how the data will be displayed (the grouping) and put it as dataset of the list:

 if (lista.Count != 0)
        {
            int cont = 0;

            string turmaAtual = lista.First().idtTurma;
            foreach (var item in lista)
            {
                if (turmaAtual != item.idtTurma)
                {
                    cont++;
                    turmaAtual = item.idtTurma;
                }
                item.ordem = cont + "";
            }
            List<string> headers = lista.GroupBy(aluno => aluno.idtTurma).Select(grp => grp.First().turma).ToList();

            lista = lista.OrderBy(aluno => aluno.nomAlu).ToList();
            alunos = lista;

            llsAlunosTurmas.ItemsSource = AlphaKeyGroup<Aluno>.CreateGroups(lista, headers,
                                            System.Threading.Thread.CurrentThread.CurrentUICulture,
                                            (Aluno a) => { return a.ordem; }, false);
        }