Thanks for doing this research, Nikola.
The warning that the article is not saved is not clearly visible, and some
students have managed to not save their articles without realizing that they have not saved them. The warning could be made more visible, but I believe that the best way of solving this would be to try to save the article by ajax, and to display the captcha on the article editing page itself.
https://bugzilla.wikimedia.org/show_bug.cgi?id=19648 has been sitting for a while. Patches welcome. :-)