Sometimes an apostrophe shows association rather than possession: a week's pay, Christchurch Girls' High School.
Strange to say, in such cases, you always need an apostrophe when the associated word is singular (a summer's day), but an apostrophe is optional when the associated word is plural (Christchurch Girls' High School).
This is a grammatical oddity that is the cause of many an argument, so let's look a bit closer.
These examples are to do with time, and they're singular: just one summer, just one week. In such cases you must always use an apostrophe.
It's my belief that in such cases, the apostrophe is there just to prevent confusion—because summers looks like a plural. A summers looks weird, so we always write a summer's day and a week's pay.
When it comes to plural examples of association, the apostrophe is optional.
'Yeeeeeeooooooooouuuuu traitor!' A blood-curdling roar emanates from our friends in the Apostrophe Protection Society. Some people believe you must always, always, always put an apostrophe in such phrases as visitors' book and girls' high school.
But the book does not belong to the visitors. The high school does not belong to the girls, as they quickly discover.
So how do you decide whether to use an apostrophe? In order of priority, choose like this:
1. Follow the rule set by the organization, for example, check the official stationery or web site of that particular girls school.
2. In other cases, follow your own style guide.
In the names of high schools, you'll find variation. This does not mean some schools are making a grammar mistake. It just means they have made a legitimate choice, or are stuck with a choice made long ago. The following names, all found on a school's official web site, are all grammatically correct.