Bar / Bat Mitzvah Planning with Doctor Simcha

I and several friends are starting to plan bar and bat mitzvahs for the coming year. We live in an area that has a small jewish population (the entire mitzvah class is only 8), and find that guests do not understand (or choose to disregard)dress codes and manners during the religious service (kids and adults wearing jeans, or revealing clothing, people drinking coffee, text messaging etc...). I quess it is a sign of letdown in Religious Service expectations. I am not a priss, but I find that drinking coffee during the service very disrespectful (and this was by a temple member!). My question is, would it be all right to include a card with the invite to gently advise people that jeans, flipflops, and bare shoulders (need for a coverup)are frowned upon, as well as food and electronic devices in the santuary during a service? Any ideas? How can it be worded so it doesnt sound preachy and stuffy?