Right ok - so is there no other information stored on the staff logins, staff shared areas and the like?

You could always create a policy to lock the PC after 5 minutes for TA's, if you're using windows 7 it won't render the machine unusable as they'll just be able to switch user, but also covers your back if they do leave it logged in?

There's no reason to just use one account for all - best bet is to have everyone sign up for an account, then invite them to a shared folder which they can contribute to. This would also allow the conflicted files to work as intended, so staff wouldn't lose changes to documents. You'd also have a history of who edited/deleted what if anything goes wrong!

You can also have different shared folders for different staff - for example SLT could have a folder that only SLT have access to, you can have a governors folder etc. using one account gives everyone access to everything.