<select class="form-control" name="class" id="class" onchange="" value="--">
    <option>--</option>
    <option>X NET 1</option>
    <option>X NET 2</option>
    <option>XI NET 1</option>
    <option>XI NET 2</option>
</select>
<input type="text" class="form-control" name="name" id="name" list="checkName" autocomplete="off" required>
<datalist id="checkName">
    <?php
        $sql_siswa = "SELECT name,class from student order by name;";
        $result = $conn->query($sql_siswa);
        if ($result->num_rows > 0) {
            while($row = $result->fetch_assoc()){
                echo "<option value=\"".$row["name"]."\">";
            }
        }
    ?>
</datalist>