Masalah Combo box bertingkat tidak tampil data ke combo box nya saat menggunakan Ajax dan Jquery

<div class="row clearfix">
    <div class="col-lg-12 col-md-12 col-sm-12 col-xs-12">
        <div class="card">
            <div class="header">
                <h2>Form Tambah Siswa</h2>

            </div>
            <div class="body">
                <form id="form_validation" name="form_tambah_materi" method="POST" enctype="multipart/form-data">

                    <label for="">Judul Materi</label>
                    <div class="form-group form-float">
                        <div class="form-line">
                            <input type="text" class="form-control" name="nama_siswa" required>
                        </div>
                    </div>

                    <label for="">Kelas</label>
                    <div class="form-group">
                        <div class="form-line">
                            <select name="kelas" id="kelas_js" class="form-control show-tick" required>
                                <option value="">-- Pilih Kelas --</option>
                                <?php
                                $database_kelas = $koneksi->query("SELECT * FROM tbl_kelas ORDER BY ID_KELAS") or die(mysqli_error($koneksi));

                                while ($data_kelas = $database_kelas->fetch_assoc()) { ?>

                                    <option value="<?php echo $data_kelas['ID_KELAS'] ?>"><?php echo $data_kelas['KELAS'] ?></option>
                                <?php } ?>

                            </select>
                        </div>
                    </div>

                    <label for="">Mata Pelajaran</label>
                    <div class="form-group">
                        <div class="form-line">
                            <select name="id_mapel" id="id_mapel_js" class="form-control show-tick" required>
                                <option value="">-- Pilih Mata Pelajaran --</option>
                            </select>
                        </div>
                    </div>

                    <label for="">Pengajar</label>
                    <div class="form-group form-float">
                        <div class="form-line">
                            <input type="text" class="form-control" name="nama_siswa" required>
                        </div>
                    </div>

                    <label for="">Foto</label>
                    <div class="form-group form-float">
                        <div class="form-line">
                            <input type="file" class="form-control" name="foto" required>
                        </div>
                        <span style="color:#F44336 ">*Tipe file JPG/JPEG dan Size maks : 2MB</span>
                    </div>

                    <input type="submit" name="simpan" value="Simpan" class="btn btn-success">
                    <input type="button" value="Batal" onclick="self.history.back()" class="btn btn-primary">
                </form>
            </div>
        </div>
    </div>
</div>

<!-- jquery v1.12.4 --><script src="plugins/jquery/jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function() {
     jQuery('#kelas_js').change(function() {
      var var_kelas = jQuery(this).val();
       jQuery.ajax({
                type: 'post',
                url: 'ajax.php',
                data: 'kelasajax=' + var_kelas,
                 success: function(result) {
                  jQuery('#id_mapel_js').html(result);
            }
     });
  });
});

****************************************************************

<?php
$koneksi = mysqli_connect("localhost", "root", "amerika1999", "db_e-learning") or die("KONEKSI GAGAL TERHUBUNG");

$Ajax_kelas = $_POST['kelasajax'];

if (isset($_POST['kelasajax'])) {
    $database_mapel = $koneksi->query("SELECT * FROM tbl_mapel where ID_KELAS ='$Ajax_kelas'") or die(mysqli_error($koneksi));

    $jumlah = $database_mapel->num_rows;

    if ($jumlah > 0) {
          while ($tampil_data_mapel = $database_mapel->fetch_assoc()) {
            echo '<option value="' . $tampil_data_mapel['ID_MAPEL'] . '" >' . $tampil_data_mapel['MAPEL'] . '</option>';
        }
    } else {
        echo '<option value="">- Mata Pelajaran Belum Tersedia -</option>';
    }
}

Ketika di console.log data ada. tapi di combobox mata pelajaran tidak ada data nya