Daddydaycare it comes down to how society regard single mothers as opposed to single fathers.
If you're a single father it seems to me that people are tripping over themselves to help and because it's unusual people take an interest and wonder how you manage. They tend to also start off with the assumption that you're a widower or there must be something seriously wrong with the ex to have abandoned her dc (because most people would expect a mother to stand by her dc no matter what so immediately they think that dad is in need of help/sympathy).
I have a work colleague who brought up his 2dd alone and he tells me that the ladies at his church rallied round for him; babysat, did school pick ups, would turn up on his doorstep with food/dinner, do his cleaning and ironing.
Whilst my own parents barely lift a finger and have to be practically begged to babysit if they ever do. However they have no trouble critising me, telling me what a mess my house is and wondering why I'm so tired even though they know I do it all on my own, working and volunteer at school, organise the PTA etc. While my married sister can't manage and needs help as her husband works shifts. So they're always running errands, babysitting etc whereas I'm 'used to it' so can just get on with it. One time her dh had to work and she didn't want to drive to a family wedding on her own with 2 dc in the car. So dad drove them there. Can't imagine him even contemplating doing that for me.
Being a single mothers seems to be something to be ashamed and embarassed about. It also seems to be felt that we made our bed so we now lie in it, or that we failed somehow by not having a partner around.
My mother especially seems to feel the need to apologise on my behalf for me being single and makes sure she tells everyone for me just in case there was any doubt 
I never used to say one way or the other but now at work when they start on about scrounging single mothers (as per our portrayal in the papers) I simply say 'what like me' and they mumble some excuse about not knowing my economic circumstanes and scuttle back under their rocks all embarrassed. So we are dealing with that stigma as well as the relentlessness of bring up children alone. And it is relentless when you have no help, no support and are left to simply get on with it.
On another occassion one of the bosses said to me (when I was struggling to pay petrol to get into work) 'what about the maintenance your ex husband pays you). Urm sory, I don't have an exhusband, never been married... He seemed genuinely shocked!
I was the only single parent at my ds's school when he started there (again a fairly small school like OP's although a bit bigger). And yes the cold shoulder and clikey groups are hard to deal with, especially when you're already feeling shunned by society generally.
Actually it started at antinatel classes when I was the only pregnant lady without a partner for support and had to answer questions on where my other half was (urm he legged it when I found out I was pregnangt saying he wasn't ready to be a dad yet). But try saying that to some poor soul who was only trying to make polite conversation!
And as ds has gone through school there are now more and more single parents, some of who have come over to me and said how bloody hard it is and they had no clue before.
So like you say these married ladies need to realise that one day it might be them and treat everyone as they would want to be treated themselves.
And I do nowadays think that the thought (that it could be them) does scare some women, usually the ones who grip their husband's arm tightly when they hear you are single. Plus there is obviously some degree of 'she's dangerous she's going to steal him away'. Not because I ever would, but some women obviously don't feel secure in their relationships and it makes them uncomfortable to be around unattached ladies. Or they're frantically trying to pair me up with anyone no matter how inappropriate someone so I can join their ranks and make them feel more comfortable.
Maybe just my perception, but it seems from what I read on this board and having spoken to other single parents I'm not the only one who gets this impression or feels the cold shoulder at the school gate.
And I am fed up with everyone assuming I'm Mrs simply because I have a child. I had to tell school that actually I'm MISS not MRS (as per the paperwork I filled in). They still call me Mrs. They just don't seem able to cope with anything that isn't 'normal'. And I think that applies to many people.
OP don't be embarrassed. You have just as much right to be there to support your dc as any of the others. And trust me you won't be the only single parent by the time your dc leave school.
I have found a couple of really good friends now. It has taken time but they are clearly secure in their relationships and have no issue with me being single. I'm sure there must be more people like that out there and hopefully if you are honest and straightforward with people they will see you for who you are, that you are genuine and a good freind to have regardless (or maybe because) of your single status.
I won't say it gets easier. But I do think it gets easier for you to handle these feelings in time as you become more confident in your single life and realise the many benefits it has for you and you dc.