How can I send an array to Codeigniter with jQuery AJAX

1. Is there another way to calculate array length on the server side?

Yes, by using sizeof($array), you can get the array length of the array.

2. Is it a good way to build an array both on the server side and on the client side?

Using name="mark-field[]" you can send the mark list without manually construct it in your javascript, also by using sizeof($array) you can get array size in the server side without sending the size from your javascript.

3. Is there another way to create and send them to the server?

Personally, I would do something like this:

<form id = "form_data" method="post">
<span>Teacher Name : </span> <input id="teacher" name="teacher" type="text">
<span>Course Name : </span> <input id="course" name="course" type="text">
<table id="students_list">
    <tr>
        <td><span>George</span></td>
        <td>
            <input name="mark[]" type="text" id="1105"/>
            <input name="mark_id[]" type="hidden" value="1105"/>
        </td>
    </tr>
    <tr>
        <td><span>Danny</span></td>
        <td>
            <input name="mark[]" type="text" id="1351"/>
            <input name="mark_id[]" type="hidden" value="1351"/>
        </td>
    </tr>
    <tr>
        <td><span>Linda</span></td>
        <td>
            <input name="mark[]" type="text" id="3486"/>
            <input name="mark_id[]" type="hidden" value="3486"/>
        </td>
    </tr>
</table>
<button id="save_marks">SAVE</button>
</form>

and the javascript part

$(document).on('submit', '#form_data', function (event) {
    event.preventDefault();
    var data = new FormData(this);
    $.ajax({
        //fill url with your controller name
        url:"<?php echo base_url(); ?>controllername/save_marks",
        method:"POST",
        data: data,
        async: false,
        processData: false,
        contentType: false,
        cache:false,
        dataType:"json",
        success:function(returndata)
        {
            //do something with your returned data
        },
        error: function (xhr, ajaxOptions, thrownError) {
            console.log(xhr.status);
            console.log(thrownError);
        }
    });
});

and in your controller:

public function save_marks() {
    $teacher= $this->input->post('teacher',true);
    $course= $this->input->post('course',true);
    //mark & mark_id is an array
    $mark= $this->input->post('mark',true);
    $mark_id= $this->input->post('mark_id',true);
    
    $arr = array();
    foreach($mark_id as $key => $row)
    {
        $arr[] = array(
            'TEACHERS' => $teacher,
            'COURSES' => $course,
            'MARKS' => $mark[$key],
            'ID' => $row
        );
    }
    
    $this->db->insert_batch('marks_table', $arr);
    //die($this->db->affected_rows() . ' marks were saved.');
    
    echo json_encode($arr);
}

By sending formdata, you wont need to manually construct the array, however since you need to send the mark_id to the controller, you need to add 1 more field to send the mark_id