WPMU privacy plugin

Hi,

I would like to ask if there are plugins which can do the following:
– users are registered per sub-blog (They have access only the blog they are registered for, not the whole network)
– all sub-blogs pages/blog posts are visible to everybody on internet but you can make specific posts/pages visible only to sub-blog users not whole network.

I found something on the net but is not working 🙁
PS: I want to make blogs for our school where students will register per blog and they will have access to main wpmu page and their sub-blog. However they will be able to see some posts from other blogs.

PSS: Or is there some other alternative?

Thanks,
Julie