Manners, please

Do you think it's okay to parent other people's children? Like, when they don't use proper manners, is it appropriate to remind them to do so? Yesterday, I was volunteering at school and it was a situation where there was plenty of opportunity to give lessons in manners. Now, I know it's bad manners to point out when others aren't using good manners. But, as a parent in a parent-teacher association, should I be a role model. Should I remind the kids that they should say please and thank you? I found myself reeling when the kids said "yeah" and wanted to tell them to say "yes, please." Or when they said "I want red", my first impulse was to say "I would like". I guess it's hard, when you are constantly reminding yourself or your own kids to use good manners to not do the same with other children. Hmmm, how would I feel if someone reminded my kids to say please? I guess I would hope that wouldn't need to happen because my precious angels would never forget their manners!