optional parameter in permalink structure

Update

You can make use of the add_rewrite_endpoint:

add_rewrite_endpoint( 'schools', EP_PERMALINK | EP_PAGES );

You can then check in the sidebar for the schools variable:

global $wp_query;

if ( isset( $wp_query->query_vars['schools'] ) ) {
    // ...
}

What is worth mentioning is that your URL will look like www.example.com/performance/name-of-performance/schools/ instead of having schools immediately after the custom post type’s name.