The code below will work, read inline comments. Paste it into your theme functions.php or plugin.
The only problem with this is the it will only work when the template is selected and the post is saved/updated and the editor is blank.
<?php
function wpse_177576_update_editor_content( $post_id ) {
if ( wp_is_post_revision( $post_id ) )
return;
$post = get_post( $post_id );
$content = apply_filters( 'the_content', $post->post_content );
if ( '' != $content )
return; // content already exists, we don't want to overwrite it.
$template = get_post_meta( $post_id, '_wp_page_template', true ); // Returns the template file name i.e. page-accordion.php
if ( $template == 'page-accordion.php' ) { // change "page-accordion.php" to the filename of your template
$content="<h1>modify this header for title</h1>
<div id="accordion">
write you content here
</div>";
}
/**
* Update the post with template content
*/
wp_update_post( array(
'ID' => $post_id,
'post_content' => $content
) );
}
add_action( 'save_post', 'wpse_177576_update_editor_content' );