public static void llenarMatriz() {
        for (int i = 0; i < 20; i++) {
            for (int j = 0; j < 10; j++) {
                if (j == 0) {
                    alumnos[i][j] = "aaa";
                } else {
                    int random = (int) (Math.random() * 50) + 50 ;
                    String randomString = Integer.toString(random);

                    alumnos[i][j] = ""+random;
                }
            }
        }
    }