要覆盖id，您可以在$select\u options收集数据的地方获取它，也可以生成另一个值，比如说它是$collected\u id

$collected_ids = array();
if ($result->num_rows > 0) {

              while($row = $result->fetch_assoc()) {
                $select_options .= '<option value="'.$row['school_id'].'">'.$row['name'].'</option>';
    $collected_ids[] = $row['school_id'];

              }
        }

是否包含“../database/config.php”；并包括'config.php'；从不同的数据库源，如果是你应该分开两个不同的资源连接，或者如@sean所说你应该改变tbl\u school rows和tbl\u department rows的变量名
我创建了$select\u options作为示例，用于保留tbl\u school的所有行，以防止您查询每个tbl\u部门行：

<div class="table-responsive">
    <?php

    include '../database/config.php';
    $sql = "SELECT * FROM tbl_school WHERE status = 'Active' ORDER BY name";
    $result = $conn->query($sql);
    $select_options = "";
    if ($result->num_rows > 0) {

      while($row = $result->fetch_assoc()) {
        $select_options .= '<option value="'.$row['school_id'].'">'.$row['name'].'</option>';
      }
    }
    ?>
    <?php
    include 'config.php';
    $sql = "SELECT * FROM tbl_department";
    $result = $conn->query($sql);

    ?>
    <table>
        <thead>
            <tr>
              <th>NO</th>
              <th>Department</th>
              <th>Status</th>
              <th>Action</th>
            </tr>
        </thead>
    <?php
    if ($result->num_rows > 0) { 
        ?>
        <tbody>
          <?php
            $no = 1;
            while($row = $result->fetch_assoc()) {
                 $session = $row['session'];
                 if ($session == "AM") {
                 $st = 'Morning';
                 }else{
                 $st = 'Afternoon';                                          
                 }
            ?>
            <tr>
                <td><?php echo $no; ?></td>
                <td><?php echo $row['department'] ?></td>
                <td><?php echo $row['status'] ?></td>
                <td>
                    <select class="form-control" id="school" required>
                        <option value="" selected disabled>-Select School-</option>
                        <?php
                        echo $select_options;
                        ?>
                    </select>
                </td>
            </tr>
            <?php
                $no++;
            }
        ?>
        </tbody>
        <?php
        }
        ?>
    </table>
</div>