Completando o cadastro

Não estou conseguindo deletar os alunos cadastrados e não consigo encontrar o erro.

package br.com.cadastro;

import java.util.List;

import br.com.cadastro.dao.AlunoDAO;
import br.com.cadastro.modelo.Aluno;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.ContextMenu;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.MenuItem.OnMenuItemClickListener;
import android.view.View;
import android.view.ContextMenu.ContextMenuInfo;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.AdapterView.OnItemLongClickListener;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

public class ListarAlunos extends Activity {

    private ListView lista;
    private Aluno aluno;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_listar_alunos);


        lista = (ListView) findViewById(R.id.lista);
        registerForContextMenu(lista);
        lista.setOnItemClickListener(new OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> adapter, View view,
                    int posicao, long id) {
                Toast.makeText(ListarAlunos.this, "Clique posicao " + posicao,
                        Toast.LENGTH_SHORT).show();

            }
        });

        lista.setOnItemLongClickListener(new OnItemLongClickListener() {

            @Override
            public boolean onItemLongClick(AdapterView<?> adapter, View view,
                    int posicao, long id) {

                aluno = (Aluno) adapter.getItemAtPosition(posicao);

                return false;
            }
        });

    }


    @Override
    public void onCreateContextMenu(ContextMenu menu, View v,
            ContextMenuInfo menuInfo) {


        menu.add("Ligar");
        menu.add("Enviar SMS");
        menu.add("Navegar no Site");
        MenuItem deletar = menu.add("Deletar");
        deletar.setOnMenuItemClickListener(new OnMenuItemClickListener() {

            @Override
            public boolean onMenuItemClick(MenuItem item) {
                AlunoDAO alunoDAO = new AlunoDAO(ListarAlunos.this);
                alunoDAO.deletar(aluno);
                alunoDAO.close();
                carregaLista();
                return false;
            }
        });
        menu.add("Ver no Mapa");
        menu.add("Enviar e-mail");

        super.onCreateContextMenu(menu, v, menuInfo);
    }

    @Override
    protected void onResume() {
        super.onResume();

        carregaLista();
    }


    private void carregaLista() {
        AlunoDAO dao = new AlunoDAO(this);
        List<Aluno> alunos = dao.getLista();
        dao.close();

        int layout = android.R.layout.simple_list_item_1;
        ArrayAdapter<Aluno> adapter = new ArrayAdapter<Aluno>(this, layout, alunos);

        lista.setAdapter(adapter);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.listar_alunos, menu);

        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {

        int itemClicado = item.getItemId();

        switch (itemClicado) {
        case R.id.novo:
            Intent proximoFormilario = new Intent(this, Formulario.class);
            startActivity(proximoFormilario);
            break;

        default:
            break;
        }

        return super.onOptionsItemSelected(item);
    }


}

classe DAO

package br.com.cadastro.dao;

import java.util.ArrayList;
import java.util.List;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteOpenHelper;
import br.com.cadastro.modelo.Aluno;

public class AlunoDAO extends SQLiteOpenHelper{

    private static final String DATABASE = "CadastroCaelum";
    private static final int VERSAO = 2;

    public AlunoDAO(Context context) {
        super(context, DATABASE, null, VERSAO);

    }

    public void salva(Aluno aluno) {

        ContentValues values = new ContentValues();

        values.put("nome", aluno.getNome());
        values.put("endereco", aluno.getEndereco());
        values.put("telefone", aluno.getTelefone());
        values.put("site", aluno.getSite());
        values.put("nota", aluno.getNota());
        values.put("foto", aluno.getFoto());

        getWritableDatabase().insert("Alunos", null, values);

    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String ddl = "CREATE TABLE Alunos(id PRIMARY KEY, "+
                 "nome TEXT UNIQUE NOT NULL, telefone TEXT,"
                 + "endereco TEXT, site TEXT, foto TEXT, nota REAL);";

        db.execSQL(ddl);

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

        String ddl = "DROP TABLE IF EXISTS Alunos";
        db.execSQL(ddl);
        this.onCreate(db);


    }

    public List<Aluno> getLista() {
        String[] colunas = {"id,nome,endereco,telefone,site,foto,nota"};
        Cursor cursor = getWritableDatabase().query("Alunos", colunas , null, null, null, null, null);

        ArrayList<Aluno> alunos = new ArrayList<Aluno>();

        while(cursor.moveToNext()){

            Aluno aluno = new Aluno();
            aluno.setId(cursor.getLong(0));
            aluno.setNome(cursor.getString(1));
            aluno.setEndereco(cursor.getString(2));
            aluno.setTelefone(cursor.getString(3));
            aluno.setSite(cursor.getString(4));
            aluno.setFoto(cursor.getString(5));
            aluno.setNota(cursor.getDouble(6));;

            alunos.add(aluno);
        }

        return alunos;
    }

    public void deletar(Aluno aluno) {
        String[] args = {aluno.getId().toString()};
        getWritableDatabase().delete("Alunos", "id=?", args );

    }

}