Blimey, these threads are always so enlightening! The bit about the PTA made me lol, reminded me of the classic "tableclothes" thread some years back.
I'm just about to start helping out in reception as a volunteer (well from Sept when I'll no longer have a child in reception) - I had no idea I'd be seen as a no-life saddo or an interfering gossip by some people! I also regularly go to school in my gym kit (or at least I did until ds3 arrived - guilty as charged ...)
I'll add it to my little list - I guess it's not quite as bad as being "wanky" for putting Ben 10 books in party bags recently!
Seriously though, I do think maybe there's an element of over-thinking this. If you can help out at school in the PTA or whatever, great. If you have people to chat to, great. If you don't want to chat then that's fine too.
In terms of the school gate, when ds1 started school I knew no-one - not one person - because we travel some distance to school and many others are very local. Still, I've made some good friends in the last two years. Sure, there will always be the odd people that blank you, but my general philosophy is to go in with a smile on your face, trill "MORNING!!" loudly to anyone that is studying their watch as you pass, and move on. Life's too short to worry about fitting in with cliques!