The group I belong to is about to buy a brass plaque bearing the group's name - FORMER PUPILS ASSOCIATION. That is my choice, with no apostrophe. Others want to call it FORMER PUPILS' ASSOCIATION. Who is right, or are either acceptable?
Both are acceptable; they just take slightly different approaches.
FORMER PUPILS' ASSOCIATION is the "possessive" form, directly suggesting that the association is of/for the pupils. This form is used in, for example, READER'S DIGEST.
FORMER PUPILS ASSOCIATION is the "descriptive" form, telling us what type of association it is. This form is used in, for example, ARTS COUNCIL, SPORTS COAT, and BOLTON WANDERERS FOOTBALL CLUB.
Which to choose? That's just personal preference. I'd go with the descriptive version, as it's less clunky. Others may not. Neither is wrong.
As an advocate of hyphens I'd offer the humble opinion that "FORMER PUPILS ASSOCIATION" could mean that the association (of pupils) no longer exists, whereas "FORMER-PUPILS ASSOCIATION" would avoid any possible confusion on that point.