We follow the usual guidelines of Is it relevant to our curriculum? Is it accurate? Is it worn out? Does it circulate?
I think some of the guidelines are deliberately vague so that the librarian can use her own judgement in deciding what to keep and what to weed. Rather than try to weed the entire collection, start with a section you think needs the most attention. Usually it's science or geography. Hopefully you will have funds to replace the weeded items, although It is better to have no information than inaccurate information.
Have you seen the CREW method?
You can break it down section by section. It is not vague and is not only a good guide for you, its a good guide in case you need to justify the weeding to administration or the community.
Don't keep 'bad' books just to have something on the shelves--your students deserve good books to choose from!