Hi E.,
She may be finding a button to push with you. I would relax on it awhile, maybe find some books on manners to read at her discretion, and put together a "manner" chart---it could have
**says hello to one friend
**says goodbye to teacher
**whatever else you want although I would not go overboard--maybe just 3-4 at the most.
Let her earn a sticker or something to put on her chart if she remembers to do what is listed. You could review the chart on your way out the door (or it could travel in the car with you). See if she does it on her own and as soon as she gets in the car to come home...hand out the stickers. I agree with not sweating over the small stuff, and at this point, I would consider this very small. As she gets older, it would not be, but right now some much development is occuring neurologically and emotionally (especially if she senses a new baby coming) that I would put the power in her court on this issue, however try to influence the progress with something motivating (i.e. the sticker chart).
Hope this helps....
K.