How to customize password protection form?

At http://www.dragontrainings.com, I’m using a child theme of twentythirteen. I’ve added a functions.php file to the child theme folder. Following advice found elsewhere, I entered and tested this function:

<?php
function my_password_form() {
    global $post;
    $label = 'pwbox-'.( empty( $post->ID ) ? rand() : $post->ID );
    $o = '<form action="' . esc_url( site_url( 'wp-login.php?action=postpass', 'login_post' ) ) . '" method="post">
    ' . __( "You must be accepted into the training to access this page.\nPlease enter the password you were given:\n" ) . '
    <label for="' . $label . '">' . __( "Password:" ) . ' </label><input name="post_password" id="' . $label . '" type="password" size="20" maxlength="20" /><input type="submit" name="Submit" value="' . esc_attr__( "Submit" ) . '" />
    </form><p style="font-size:14px;margin:0px;">∗EXTRA TEXT CAN GO HERE...THIS WILL SHOW BELOW THE FORM</p>
    ';
    return $o;
}
add_filter( 'the_password_form', 'my_password_form' );
?>

This code, which I think predates 2013, doesn’t work, and it breaks the wp-admin page, getting “already loaded” errors. So, I had to rename the functions.php file to get back into the site.

Suggestions welcome!