Is it my fault?

I left school in December and I've started to notice that because I've left none of my friends/best friends want to take anything to do with me. They all do stuff together and I feel quite left out. I keep thinking that it's my fault but I do try and make an effort to be friends with them just they don't with me. We have all just drifted apart and it's like they have forgotten about me and it does get me down because my social life has basically gone because they were my mates and I feel mate-less. I haven't got a scooby if what I said made any sense but I really don't know what to do. I don't want to pester them about it.

It probably has something to do with you leaving school like they might disagree with your reasons, I know that is no reason for them to start leaving you out and stuff because that's just unfair and if that is the reason they should just talk to you about it. Have you confronted them and asked them why they're not talking to you and making the effort with you because you could maybe do that because they may not be realising what they're doing and they might lose track of things. Hope this helps :)

I hate to say it, but we all told you that your social life would go down hill etc..

i only know about four people who still remain in the same group they were in once they left, and none of them have an intact group.

it's part of growing up, they have things in common: school. You don't.

Find new friends, move on.

Arrange to meet up with them all, if some of them cancel then confront them, not horribly, but in a calm way. Something like "Is there a problem? I feel like we're drifting apart..." if they come up with a reasonable explanation, then obv they wanna be your friend still but they maybe just feel abandoned.
One of my mates got expelled last year and some of the lads in our group stopped speakin to him, but I didn't. They were angry that he'd done something stupid and got himself kicked out. There's always a reason.

Few friends of mine i've sorta drifted apart with as they didn't go to college after secondary and i just seem to have lost contact with them.

As they are all together, they just won't think about you - it wont be personal they just have things going on in their lives which means they accidently forget about you.

To answer the question you asked;
Yes.

it happens what did you expect, they see each other every day cos of school, if you're not there you're obviously gonna miss out

It happens unfortunately, and to quote the cliché, it's just part of growing up. I wouldn't say it's your fault, but if you want to stay in contact with them, I would tell them how you're feeling about it.