Hi Choochoo,
It is entirely normal to feel a little anxious when your eldest starts school. It is a whole new chapter in his, and your, life!
I think it is very normal for 3-4 year olds to not exactly join in at parties very much. Both of mine were like that - and are now socially very open and confident. I wouldn't take that as a sign of your 'social inadequacies' affecting him. It is just his developmental stage! Remember, they all develop at different speeds in different areas, AND he is younger than most of his peers, so there is absolutely no point in comparing him to his friends.
Yes, chances are there will be mums who know each other very well already, and it can seem 'cliquey' - sometimes it is, sometimes it only appears that way.
However, remember that there will be plenty of other 'new' mums at the school. And they will be feeling just as nervous as you! At the settling in sessions/information evenings/other similar events, if you spot someone standing around by themselves a little awkwardly, why don't you just go up to them and say something like:
"Hi, I'm x, my boy y will be in reception, are you new to the school too? I must admit, I'm rather nervous, more nervous than my little one in fact haha!"
And then ask about their children, people are always happy to talk about their children :)
Two more things:
- There will probably be a class e-mail-list/whatsapp group/facebook group. Get yourself on there. Put people's names along with their child's name in your address book, so whenever a message pops up it reminds you of which parent goes with which child e.g. Jenny (Georgina) ...
These groups are great for initial contact, and staying in the loop for what's happening.
- Though you will probably be seeing these people for a fair few years, they don't need to be your friends. You don't have to socialise with them! There is nothing wrong in just dropping your child off and leaving right away, and arriving just in time for pick-up and leaving straight away. You might meet some lovely people, and it is probably worth the effort to at least try (for occasional help with pick-ups if you have been delayed, shared lifts, that kind of stuff) - but if these people remain nothing more than 'acquaintances' to you, that is fine too and doesn't mean that there is anything wrong with you at all. Maybe remembering that you don't have to become 'friends' with these people (if you do, it is an added bonus) will help you relax a bit about it all?