Deregister WordPress jquery on specific page

Hook to wp_enqueue_scripts. It will do the trick.

Modified code is-

if (!function_exists('modify_jquery')) {
    function modify_jquery() {
        if (is_page(array('page 1', 'page 2'))) {
            wp_dequeue_script('jquery');
            wp_deregister_script('jquery');

            wp_register_script('jquery-custom', '//ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js', false, '1.11.3', 'true');
            wp_enqueue_script('jquery-custom');
        }
    }
}
// add_action('init', 'modify_jquery');
add_action('wp_enqueue_scripts', 'modify_jquery');

Hope this is gonna help.