Hi Hirsty,
Are you on different pc's each time you visit from school? What browser are you using?, and do they clear the cookies off the pc's you're using periodically? Do you have your own network id to log into? If the cookie is deleted, or if the security settings of the browser you're using are set not to store them, then you will not stay logged in. If you're using the site from a public computer network, you probably don't want the board to keep you logged in anyway, or someone who uses that machine after you would be able to access your account, because they'd be logged in as you.. should they find the site by browsing from the history.
The next version of this forum software has a 'quick login box' located at the bottom of all the forums. We will upgrade to the final version of it sometime after it's released. It's currently in beta. I'm not sure if it will be much help to you, because it will require you scrolling all the way to the bottom of the forum to login. It'd probably be easier to use the current method of logging in even after we upgrade. Running off public pc's is less convenient, but there's not much we can do to help given the security concerns of browsing from a shared pc.
Sorry