So i'm (about to be) in the same boat - single parent, 40.
I haven't yet found myself excluded - but maybe that's because I'm not quite yet in the school circuit..... it also helps that of the relatively small circle of friends I have these days, some have known me for 15 years+, before I was married and therefore are interested in looking after me in this shitty time.
I do find already though that I don't necessarily want to intrude on my friends' happy family weekend time, so find either weekends with my kid or without, are lonely and loooong. I likewise make multiple weekend plans to keep busy - but it often feels like a lot of effort to travel to friends with the kid in tow.
Sooo.... like the pp, I'd be happy to meet up for a coffee if anyone's keen. I know you are asking for advice, not begging for friends - But I've felt like this before for different reasons (pre marriage and pre kids) and just a couple of decent friends can make a real difference. Please pm me if keen. Am mostly around over the holidays