If you have to browse BOS at school, make sure you only browse from 1 computer, at any one time.
I use to browse BOS at school, but I would only use *my* computer at school, allocated to me. That way, even if I store login information (which I never did, disabling cookies), then if I returned to the site, you would still have to login before you can post.
My suggestion for you would be to:
* Make sure no one touches your computer, but if it is freely available, don't store your username and login information (or turn off cookies if possible). Clear the cookies and log out of BOS instead of simply clicking X (close). That way, nobody can simply return to BOS, find out that someone has been browsing and posting, and post under your name (masquerading as you).