I think perhaps a lot of people feel that they have two or more different "lives" or "personalities", depending on who they're with. Basically, you interact differently with people according to who they are. Perhaps you're more respectful to your teachers than to your parents - your teachers might be quite amazed at how brattish you sometimes act with your parents! Maybe if you feel sad when you're at home, you're more likely to let it out than when you're with your friends, because you don't want to seem weedy or whatever to them. It naturally follows that you'll act differently around different sets of friends. Perhaps it just so happens that you feel more confident around your out-of-school friends than your school friends, and so you walk tall, smile nicely and look like a bit of a stunner out of school, and hunch your shoulders and don't say much while you're in school. That's fine - enjoy both sides of your personality. When you're confident, revel in it, when you're quiet, enjoy the peace you get.
Alternatively, it could be that your schoolfriends have known you for longer than your out-of-school friends. They remember you when you were a scrawny little Year 7 dude, then a grunty Year 9 and so your transformation or whatever into ultra-cool 6th former has sort of gone unnoticed. Whereas when you're at work, most of your workmates will have known you only for a few months or so, so you're more of a novelty to them and in turn, you find them more interesting.
Whatever the reason, it happens to most people, so don't worry!
xxx