What practical method would you suggest to get them to follow the requests?
Have the Rebbitzen first call, then go to each one privately in their own home, and offer to discuss the Halachos of Tznius inside a real sefer (such as the Mishna Berurah). Many people don’t follow Tznius because either they think it is made up (blame the schools that really do make up Halachos) or they believe it is muttar (i.e. the famous misread Aruch Hashulchan). If you do it in public, it will never work, and it will boomerang so that they will not listen even if you are nice about it the next time.
Hatzlacha.