I'll cut to the bottom line first on this one. The friendships you make at your childrens' school can be life-enhancing and a fantastic source of support. Making friends with other parents is a win-win situation. Your kids will benefit because other parents will encourage the childrens' friendships (especially in the "play date and parties" years) if they like you and your partner. You win, because you are meeting and liking new and different people and having very local friends enhances a sense of community.
However...it can sometimes take a while to suss out who you are going to choose as your friends. I read an article once about this and the writer made what I think is a very valid point. When we are at school, there is no compulsion for anyone to like us. In adult life and the workplace, people are forced to engage and therefore once they get to know us (because they HAVE to), they start to like us and they therefore forget that their initial impression of us wasn't that great.
Even in other adult situations, there is a compulsion of some sort for people to get to know us (adult courses when you have to collaborate, mutual friendships etc.). However, when we are parents in a school playground, this is the one situation where as adults, there is absolutely no compulsion to get to know anyone and like them. We therefore revert to sticking with our first impressions of people and not moving past them. In a completely non-sexual way, we "fancy" the look of some parents and not others - and we don't question that at all.
Into the mix then, we have some heady stuff about our own view of ourselves and our previous experiences when we were at school. If you enjoyed school, felt that by the end of it you fitted in very well and made good friends then and since, your approach to all this is going to be positive. If on the other hand you had that feeling of not "fitting in" when you were at school - that people didn't really "get you", or worse still, you were bullied by other girls, you are subconsciously taking that into the playground with you now. In some ways, it's even worse because you know instinctively that this may affect your child fitting in too.
The other thing to consider is the impression you are giving these women. You say they live on an estate and you live just off it. You take great care with your appearance - and although you don't say it - I get the impression that they don't? We may therefore have a tricky combination of THEM feeling insecure and wary of you - and you feeling superior in some way to them.
At the moment, I suspect this is not in your imagination at all. When my oldest started school, I remember only too well a group of women who really thought they were the bees knees and seemed to compete about which one was the "best mother". They went on every school trip, helped out at the school every day, spent hours in the playground before and after school discussing other children and their family situations in a "Tutt Tutt" kind of way. One friend of mine was often so hurt by their behaviour - on their own they would talk to her quite happily, but they would then freeze her out if one of their group turned up.
They never bothered me, because I've always been pretty secure in myself. I had absolutely no desire to become friends with them anyway, because they weren't earthy or passionate enough for me. They all seemed a bit brittle and "stepford wifeish" to me. Fortunately, my oldest never much liked their kids either, so there was no "compulsion" to make friends!
I made a few mistakes along the way too though. I was flattered at being wooed by the Chair of the PA who seemed to be the absolute Queen Bee of the playground and unfortunately, her horrible son really liked mine, so I encouraged that friendship for my own sake really -and not my son's. After a while, I realised that this woman spent her whole life gossiping about others in the playyground and she told me confidences about people that should never have been shared.
I sussed out pretty early on that I would never tell her something I wasn't happy for the world to know and I will always be grateful for that early wisdom. Eventually, I sussed out that all this PTA activity and obsession with school life was really a mask for this woman's own rubbish marriage. When she started telling me horrible stuff about people I'd grown to like - and I stuck up for them - our friendship naturally waned. God knows what she said about me, I'm sure she used to make stuff up half the time. It seemed pretty improbable to me that the playground was full of alcoholics, wife-beaters, porn stars and prostitute users!
In those primary school years, I made some great friends and we remain so to this day. We'd have been friends anyway, you see. It took a while to sort out the wheat from the chaff, but my demeanour with all of the parents was friendly right from the start.
So keep being friendly. Give it time and eventually, people will find they "have to" get to know you through the children. They will then realise that their first impressions were wrong and they might even forget what they were. At the moment you see, they just don't "fancy you", but they will in time, once you've all got rid of the complex baggage we all bring when we start at our childrens' school gates.