your post reminds me a lot of how i felt when i first moved here. (circumstances completely different but the feeling, i imagine is much the same). i have always found it very hard to make friends. im very shy. when i found myself on my own with ds i was desperately unhappy. i hardly knew anyone around here. (my own friends only live a 10 min drive but thats a lot of buses and taxi fares when you cant drive!).
people advised me to go to groups, start a course etc..but i never had the courage, it just all seemed such a forced desperate attempt to make friends that i never bothered. (and quite frankly it terrifies me).
so anyway, i moved into this street just over five years ago and i used to watch the neighbours all chatting happily to each other and sitting in each others gardens. one of them approached me when i moved in but that was it really. i thought they were all "in" with each other and i didnt ever think they would like me.
so i kept myself to myself mostly and i was lonely for a very long time. last year i had to speak to one of the neighbours (some mixed up post or something) and she kind of attached herself to me...now, although she really annoyed me for about a year (she was just too much), when the other neighbours saw that i spent time with her, they started coming over and speaking to me aswell. i didnt quite understand why at the time.
it all evened itself out and its nice now knowing that if im bored, i have a choice of a few people i can go and talk to or invite over. i do a course with one, we take the children out with some of the others and we all generally have a good gossip.
its like the typical "can i borrow a cup of sugar" thing here now but its taken a long time to get there!
the neighbour who im more friendly with now is the one i originally thought was the "main" one who everybody wanted to talk to. she admitted some time ago that she never bothered to talk to me as she thought i was stuck up. when i asked why, she said it was because i never came over to them and joined in. i explained that i was too shy and thought i would be "intruding" on what i saw as their little gang. she told me not to be so silly!
so while, i STILL would not approach a group of women who all obviously knew each other really well, (im too frightened), it just shows how we got totally the wrong idea about each other. she's actually very nice and helps me out a lot.
so i suppose what i would do, being the person i am (no confidence whatsoever) is probably not try and join in with a group of mothers chatting, (i would feel very uncomfortable with that), but if you pick the one who you see as the most approachable, when you see her on her own, then go over and have a chat and over time you will probably get to know all the others through her.
(you may find that they will make an effort to talk to you even if its mainly because they want to be nosey!! ...but of course, thats how you get to know people and how friendships start)
im using my street as an example and your street is probably very different to mine but i imagine that would prob work the same at the school. when you get to know one of them, the others may be more interested.