I’m easily irritated too, and I love that you’re checking with MN! I should do that more often myself.
One thing that has helped me with ‘school mum’ situations like this (and boy have I had more than my fair share) is to keep things fairly ‘bright n breezy’ from start to finish. That is, be kind and helpful, but not properly befriend them or become their best/main support.
It’s so easy to fall into, cos you have so much in common, you immediately empathise with the whole ‘life with kids’ thing, or the fun low-level bitching about teachers or hubby. You realise you see so much more of them (twice a day!) than your real life friends, and it just feels SO GOOD to fast track friendship with them.
Then you discover that (no harm, no foul), you actually have very little in common with them. That their values or behaviours are nothing like yours. That if you’d met them as work colleagues or at the pub, you would not have given them a VIP pass to your life.
Now you’re stuck finding ways to down-grade the level of intimacy without hurt feelings. You feel your heart drop a little bit every time they approach you. Twice a day, for years. That is tricky, awkward and depressing (for everyone).
Of course some people find life-long besties in the school pickup queue. But I tend to treat that queue as a cruiseship rather than a lifeboat, if you see what I mean.
Because of my middle-aged impatience, I don’t mind being fairly clear these days when lines get crossed. This week, instead of wringing my hands over a response to a school mum (and historical cf), I just replied ‘I’ve thought about it, and I just think our values are too different as families for that arrangement to work.’ I said it with a real smile, full eye contact. I really meant it. Said it, then changed the subject to a lighter topic.
I recommend doing this when she next asks for something unreasonable, before you (and DH) build up a head of steamy resentment. Clear boundaries help everyone, and proper dramatic fall-outs at schools are to be avoided if at all possible!