functions.php

/** Register connection type (required for Posts 2 Posts plugin
https://github.com/scribu/wp-posts-to-posts/wiki/Basic-usage  */
function my_connection_types() {
  p2p_register_connection_type( array(
		'name' => 'classes_to_events',
		'from' => 'class',
		'to' => 'event',
		'cardinality' => 'one-to-many'
	) );
}
add_action( 'p2p_init', 'my_connection_types' );

/** Modify output for connections
https://github.com/scribu/wp-posts-to-posts/wiki/Actions-and-filters */
add_filter( 'p2p_widget_html', 'my_p2p_template_handling', 10, 4 );
add_filter( 'p2p_shortcode_html', 'my_p2p_template_handling', 10, 4 );
function my_p2p_template_handling( $html, $connected, $ctype, $mode ) {
	$template = locate_template( "p2p-{$mode}-{$ctype->name}.php" );

	if ( !$template )
		return $html;

	ob_start();

	$_post = $GLOBALS['post'];

	foreach ( $connected->items as $item ) {
		$GLOBALS['post'] = $item;

		load_template( $template, false );
	}

	$GLOBALS['post'] = $_post;

	return ob_get_clean();
}