Recently active topics in PBwiki tagged with security setting

Hey,
I have some problems getting my wiki to save the secutiry settings. I have made my wiki public because I want everybody to have access to the frontpage and the files I have linked to on the frontpage. But then I want to have another layer at the wiki, where it is only possible to get access if you are logged in as a user of the wiki, at this page I have made the security settings to 'custom' and then added users.
But the security setting keeps on switching back to "default" even though I am absolutely sure, that I have pressed save. And I have tried switching the setting back to custom 4 or 5 times, so I am really confused about what I am doing wrong...

So now I am forced, to protect the files in the non-public part of the wiki, to make the whole wiki private, with the result that the images and files on the frontpage do not work...

Can you help me?

PS. Is it possible to create a new wiki, and make that the public part of my site, and then create a link to the private wiki?

I have an educational free wiki and cannot change the security level of pages or folders. Is it possible? It says I have to upgrade to change security. Does this mean that when I try to lock pages so that my students cannot edit them any more, I can't do it unless I upgrade?