You can simply create the pages and remove them from the menu - see:
Thanks, this did the job; I created a custom menu and added the pages I want to appear in the menu. Excluding the page I did not want in the menu in this custom menu quite naturally did the job:-)
the best and easiest way is as @WPyogi suggested.
if you want to customize the theme to exclude pages from the top menu,
start by creating a child theme of Twenty Twelve; http://codex.wordpress.org/Child_Themes
Twenty Twelve uses 'wp_page_menu()' as a fallback if the custom menu is not activated; this needs to be influenced in functions.php;
add a new functions.php into the child theme's folder; make sure to have the <?php at the start;
add this code (the numbers are the page IDs of the pages you want to exclude)...
I already made a child theme, so thanks also for this suggestion. Is this in any way a better way to exclude pages?
As I see it, when all I want is to exclude one or more pages, the procedure described above is simpler. But if more changes, e.g. customization of the design of the menu, is needed the changes in functions.php are required. Is that correct?