Dúvida no Ex. 6 da Aula 4 - Tela de Cadastro

Bom o meu menu não está abrindo na tela de lista de alunos, eu não estou conseguindo achar o erro, estou dando inflate corretamente mas não aparece, segue as minhas classes:

Manifest:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="cadastro.caelum.com.br.cadastrocaelum">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme">

        <activity android:name="cadastro.caelum.com.br.cadastrocaelum.ListaAlunosActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name="cadastro.caelum.com.br.cadastrocaelum.FomularioActivity"></activity>

    </application>

</manifest>

menu_principal.xml:

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/menu_novo"
        android:icon="@drawable/ic_novo"
        android:title="Novo"
        android:showAsAction="always" />

    <item
        android:id="@+id/menu_mapa"
        android:icon="@drawable/ic_mapa"
        android:title="Mapa"
        android:showAsAction="always"/>
    <item
        android:id="@+id/menu_opcoes"
        android:icon="@drawable/ic_opcoes"
        android:title="Mais opções"
        android:showAsAction="always">
        <menu>
            <item
                android:id="@+id/menu_enviar_alunos"
                android:icon="@drawable/ic_enviar"
                android:title="Sincronizar"
                android:showAsAction="always"/>
            <item
                android:id="@+id/menu_receber_alunos"
                android:icon="@drawable/ic_receber"
                android:title="Baixar Provas"
                android:showAsAction="always"/>
            <item
                android:id="@+id/menu_preferencias"
                android:icon="@drawable/ic_preferencias"
                android:title="Preferências"
                android:showAsAction="always"/>
        </menu>
    </item>
</menu>

ListaAlunosActivity.class:

public class ListaAlunosActivity extends Activity {

    private ListView listaAlunos;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.listagem_alunos);

        String[] alunos = {"Anderson", "Filipe", "Guilherme"};

        listaAlunos = (ListView) findViewById(R.id.listagem_ListView);

        final ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, alunos);
        listaAlunos.setAdapter(adapter);

        listaAlunos.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Toast.makeText(ListaAlunosActivity.this, "Posição seleciona: " + position, Toast.LENGTH_LONG).show();
            }
        });

        listaAlunos.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
            @Override
            public boolean onItemLongClick(AdapterView<?> parent, View view, int position, long id) {
                Toast.makeText(ListaAlunosActivity.this, "Nome do aluno: " + parent.getItemAtPosition(position), Toast.LENGTH_LONG).show();
                return true;
            }
        });

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        getMenuInflater().inflate(R.menu.menu_principal,menu);

        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        return super.onOptionsItemSelected(item);
    }
}