This is a move that could work either way for you (it could work out very well or not-so-well).Research your choices... Extended high school 6th form:-
if you go to a sixth-form that's part of a "middle-school
", most people will be from that school (there won't really be many "newbies
". You say you've got friends in other schools? So that could work out for you if you talk to them and see what they're planning on studying and where they want to go. On the other hand, if you go alone, most people will be from that particular school and will have their own groups, cliques etc. so you risk having the same problem. Fair enough, they may be a bit more grown up and work focused, but you may still struggle to find some friends. Dedicated 6th Form college:-
Mainly same / similar sort of people, but they may be slightly more open-minded than those going to the sixth form as part of their school. There will most likely be groups of people from other schools, but there will be a few who are there by themselves... although I don't know how easy it will be to find them.
County / borough college:-
This will probably be the fastest learning curve, and there will be several people who will have gone there by themselves (not part of an existing group/ clique).. Everything is on a much wider scale and they offer a much wider range of courses (including many vocational courses; as opposed to the traditional "A" levels). You don't really have tutor groups as you might do in 6th forms, and the people are lecturers (as opposed to teachers). They won't push you in the same way school teachers might, so you need to show some focus, discipline and initiative. The other thing about college is there's no maximum age limit (6th forms tend to have an upper age limit of 18-19). That may or may not suit you, but IMHO, it offers a greater wider learning experience than you would get in 6th form.
Of course, in the meantime, you need to get your head down and get your GCSE's. The silver lining is that at least your school mates are less likely to distract you from your studies