Ha, ha, ha – I was about to say that you should wear jodhpurs tucked into high flat boots and a hacking jacket – but then she’d know for sure that you Google’d her!
I would not wear jeans. I would wear “nice sportswear.” This might mean pants such as ponte knit or some other sportswear styles. If you like the idea of jeans, then maybe a pair of velveteen ones. I would pair it with a very nice top or sweater and jewelry. OR, I would wear a skirt with nice top or sweater, or a fairly casual dress with tights and boots/booties/shoes (which don’t have to be dressy styles).
The link should take you to Louise Penny wearing a comfy sweater. I have some ankle high marroon lace up suede boots I love if weather is okay. My tall boots worked fine for a hipster wedding, seem wrong here. I look good in the Modcloth dresses cut like this: http://www.modcloth.com/shop/dresses/ill-take-you-flair-dress, but I am always cold so wouldn’t wear anything like this without the appropriate cardigan.
I’m not wearing jeans. Too risky, that I’d be the only one.
@JustOneDad I gotcha. Here you go.
No jeans. And if it were me, I would go shopping at Chicos. Black or tan slacks, solid top, either knit or woven, and an “interesting” jacket. Interesting might mean leather … Not a preppy blazer. It could mean a structured cardigan. And either a fabulous scarf or a unique necklace. (I’m a few miles away from back country Greenwich – our weather right now is perfect fall, temps in 60s and sunny gorgeous.)
mathmom. you will look lovely no matter what.
No suggestions except dress nicer than you think it might be. You can’t go wrong.
Just make sure your best tiara is polished. You will be good to go.
@JustOneDad it was and remains a girl’s school. No boys allowed! But if you were to come, it would be English all the way. What was with that tie with the tweed jacket in @sax 's link?
Not a charity lunch except to the extent that it’s selling the school. Generally what will happen is people schmooze around and talk to each other, we eat lunch, the current headmistress tells us about all the wonderful things that are going on in the school. We go home inspired and tell all our classmates to donate money to the school because they are doing wonderful things. It’s all about generating warm and fuzzy feelings about the school.
Hm, I do have a nice black leather jacket - cut like a blazer not like a bomber jacket. Hadn’t thought about it…
Oh, an Advancement event of alum. No to the leather jackets, no to the jeans. I like cnp thoughts on tan slacks as it seems that those are more to your colors. I would have to see the maroon boot, if you pair tan slacks with either a creamy color sweater and a scarf with maroon in it, it could work. If the weather seems like it would be warm, go with a cream background dressy blouse with maroon. Simple, understated jewelry.
Not going to link styles, but agree with black slacks (or navy, which bridges) and a cardigan or hip pullover. I’ve been to these things and am always surprised that some won’t be dressed well at all. You pretty much pick which image you want to convey (wealthy, accomplished, artsy, still sexy after all the years, whatever you feel) and go from there. Wear your confidence.
Btw, also thought of mathmom as sort of LP looking. Smart, empowered, her own self.
If you are doing the black slacks I would pair it with something like this:
If you decide to go with jeans I would dress it up a bit:
https://s-media-cache-ak0.pinimg.com/236x/39/ff/63/39ff634849d9a0d3867e0eb5d3686345.jpg
Here are some ideas from Chicos: http://www.chicos.com/store/product/neutral-paisley-andrea-long-cardigan/570153272?color=183&catId=cat40036&fromSearch=true&scPos=1-1117-3418
ECMother, as someone of mathmom’s height, I cannot see myself wearing those pieces without heels, especially the long cardigan. 
Unless there is a pair that looks like dark trousers, I would avoid jeans. Black pants, crisp white blouse, and a cardigan/jacket of some sorts would create a polished yet practical look.
(And now I know that the person wearing cargo shorts and a wifebeater to the symphony was JOD. 
).
I have an LL Bean jacket not too dissimilar from the Ralph Lauren one. I think it makes that model look sturdy and ditto with the LL Bean one. The jeans look, even with a jacket looks way, way too casual to me.
Yes, @lookingforward, there will always be someone who is really dowdy, but I don’t want it to be me!
I usually try to look trustworthy and competent and a little bit artsy. At least that’s what I think an architect should look like.
Someone earlier suggested looking at Chico’s.
Mathmom: what about something like this? With different jewelry!
:-S
I would do sweater, skirt and boots.
You can add a belt over the sweater depending on the style.
Since you are still very trim, wear dress, skirt as most other “older” people can’t pull it off.
Greenwich? My take is, unless one is visiting the stable or riding a horse don’t wear horsey type clothes. One would be play acting. In New York and Los Angeles a simple black, blue dress, blazer and boots or shoes with a bit of a heel. Dress pants, sweater set or as Bunsen said…crisp white shirt. Ann Taylor, St. John, Lafayette 148…conservation but can be dressed up or down.
I like the cardigan in #34 and I am not a Chico’s fan. You could do a lot with that. Agree the necklace is awful.
I liked that cardigan too, but almost nothing else on their site. Not sure why.
Mathmom:
Now you have me looking instead of working! 
If you think the blazer in #34 may be too structured for you, what about something like this one?