O Toast mostrando a média dos alunos não funcionou...mensagem em branco

Aparece o circulo do Toast mas sem nenhuma mensagem. Havia feito o teste antes e estava enviando os dados como o nome do aluno e a nota. Não achei o que pode estar errado.

public class WebClient {

    public String post(String json) {

        try {
            URL url = new URL("https://www.caelum.com.br/mobile");
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setRequestProperty("Content-type", "application/json");
            connection.setRequestProperty("Accept", "application/json");

            connection.setDoOutput(true);
            connection.setDoInput(true);

            PrintStream outPut = new PrintStream(connection.getOutputStream());
            outPut.println(json);

            connection.connect();

            Scanner scanner = new Scanner(connection.getInputStream());
            String responseServer = scanner.next();

            return responseServer;

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return null;

    }
}

public class SendStudentTask extends AsyncTask<Void, Void, String> {

    private Context context;
    private ProgressDialog dialog;

    public SendStudentTask(Context context) {
        this.context = context;
    }

    @Override
    protected String doInBackground(Void... params) {

        StudentDao dao = new StudentDao(context);
        List<Student> students = dao.searchStudents();
        dao.close();
        StudentConvert convert = new StudentConvert();
        String json = convert.convertToJSON(students);

        WebClient client = new WebClient();
        String averegeGrade = client.post(json);

        return averegeGrade;
    }

    @Override
    protected void onPreExecute() {
       dialog = ProgressDialog.show(context,
                "Aguarde", "Enviando Aluno", true, true);
    }

    @Override
    protected void onPostExecute(String averageGrade) {
        dialog.dismiss();

        Toast.makeText(context, averageGrade, Toast.LENGTH_LONG).show();
    }
}

`