จะให้กด checkbox แล้วสามารถเลือก dropdown ได้ ถ้าไม่ติ๊กไม่สามารถเลือกได้ ข้อมูลดึงจาก mysql มาทั้งหมดเลยคะ

<script type="text/javascript">
function check(e){
if(e.checked == true) {
document.getElementById('UserID_teacher[]').disabled=false;
} else {
document.getElementById('UserID_teacher[]').disabled=true;
}
}
</script>

....


<table width="26%" class="table table-striped table-bordered table-hover">
                      <thead>
                                        <tr>
                                            <th width="17%">รหัสรายวิชา</th>
                                          <th width="33%">ชื่อรายวิชา</th>
                                          <th width="17%">ประจำชั้น</th>
                                          <th width="12%"></th>
                                          <th width="21%"></th>
                                          
                        </tr>
                                  </thead>
                                    <tbody>
                                    
                                    
                                       <?php
		  if($_GET["item"] != "")
		  {
			$strSQL = "SELECT distinct course.course_id, course.course_name, class_student.Class_name  FROM course, class_student
WHERE course.Class_ID='".$_GET["item"]."' 
AND course.Class_ID=class_student.Class_ID";
			$objQuery = mysql_query($strSQL);
			$item = $_GET["item"];
					while ($row = mysql_fetch_array($objQuery)){
							echo "<tr><td>". $row['course_id'] ."</td>
									<td>". $row['course_name'] ."</td>
									<td>". $row['Class_name'] ."</td>
									<td><center><input type='checkbox' id='course_id[]' name='course_id[]' value='". $row['course_id'] ."' onClick='check(this)'/></center></td>
									<td><select class='form-control' id='UserID_teacher[]' name='UserID_teacher[]' value=". $row1['UserID_teacher']." ><option>-- กรุณาเลือก --</option>";
									
									$sql = mysql_query("SELECT * FROM member_teacher");
									while ($row1 = mysql_fetch_array($sql)){
									echo "<option  id='UserID_teacher[]' name='UserID_teacher' value=". $row1['UserID_teacher'].">" . $row1['TeacherName'] . "</option>";
									}
									
							echo "</select></td></tr>";	
							}                                                                                                                                            
				}
			
		  ?>
                      
                      
                          <tr><td colspan="5"><div align="center">
                       
                       <button type="button" class="btn btn-primary" onClick="history.back();">Back Button</button> 
                         <button type="submit" class="btn btn-primary">Save Button</button>
                       
                     </div></td></tr> 
                      </tbody>
                      </table>