WordPress Save Dropdown Metabox - php

Question

I have already created custom meta box at my custom post type.
so I create drop down list to choose a Teacher's name and the Class.
The value can be shown in the dropdown, but after publishing the post the value cannot save and dropdown set to default again.
can somebody know, if I am missing part or wrong code here?
Thanks if you wanna help, so here's my code:
function portfolio_student(){
add_meta_box( 'portfolio_student', 'Select The Teachers and Classes', 'meta_box_data', 'portfolio', 'normal', 'high' ); }
function meta_box_data(){
// $post is already set, and contains an object: the WordPress post
global $post;
$values = get_post_custom( $post->ID );
$teachers = isset( $values['the_teachers'] ) ? esc_attr( $values['the_teachers'] ) : '';
$classes = isset( $values['the_classes'] ) ? esc_attr( $values['the_classes'] ) : '';
// We'll use this nonce field later on when saving.
wp_nonce_field( 'my_meta_box_nonce', 'meta_box_data' );
?>
<p>
<label for="the_teachers">Teacher</label>
<select name="the_teachers" id="the_teachers">
<option value="0">-- Select Class --</option>
<?php
$args = array(
'numberposts' => -1,
'post_type' => 'teacher'
);
$teacher_posts = get_posts($args);
foreach( $teacher_posts as $post ) : setup_postdata($post); ?>
<option value="<?php echo $post->ID; ?>" <?php selected($teachers, $post->ID); ?>><?php the_title(); ?></option>
<?php endforeach; ?>
</select>
</p>
<p>
<label for="the_classes">Class</label>
<select name="the_classes" id="the_classes">
<option value="0">-- Select Class --</option>
<?php
$args = array(
'numberposts' => -1,
'post_type' => 'class'
);
$class_posts = get_posts($args);
foreach( $class_posts as $post ) : setup_postdata($post); ?>
<option value="<?php echo $post->ID; ?>" <?php selected($classes, $post->ID); ?>><?php the_title(); ?></option>
<?php endforeach; ?>
</select>
</p>
<?php}
add_action( 'save_post', 'cd_meta_box_save' );function cd_meta_box_save( $post_id ){
// Bail if we're doing an auto save
if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
return;
// if our nonce isn't there, or we can't verify it, bail
if( !isset( $_POST['meta_box_data'] ) || !wp_verify_nonce( $_POST['meta_box_data'], 'my_meta_box_nonce' ) )
return;
// if our current user can't edit this post, bail
if( !current_user_can( 'edit_post' ) )
return;
// Make sure your data is set before trying to save it
if( isset( $_POST['the_teachers'] ) )
//Update Meta Data
$teacher_meta = $_POST['the_teachers'];
//EndUpdate
update_post_meta( $post_id, 'the_teachers', $teacher_meta );
if( isset( $_POST['the_classes'] ) )
//Update Meta Data
$class_meta = $_POST['the_classes'];
//EndUpdate
update_post_meta( $post_id, 'the_classes', $class_meta ); }

Answer 1

Try changing these
update_post_meta( $post_id, 'the_teachers', $teacher_meta );
update_post_meta( $post_id, 'the_classes', $class_meta );
To
update_post_meta( $post->ID, 'the_teachers', $teacher_meta );
update_post_meta( $post->ID, 'the_classes', $class_meta );
Since you have declared $post as the global variable