As you say that you only have a week left of school I would be very shrewd and make an effort with your existing friends. However if any one person outside your group makes an effort to talk to you then don't ignore them.
Personally I find the situation that you are in very archaic and pathetic but hey its common knowledge that kids can be cruel.
Come september I would make an effort to integrate, it will be hard but its obvious that you want to and I think you could be happier.
If it makes you feel better try making the effort away from the prying eyes of your 'group' for the first couple of months if possible, to make it easier for you.
It infuriates me how these situations can arise, segregation is just a snowball for worse things, thats how Hitler came to power for goodness sake.
Well thats my rant over. lol ;-)
Look make yourself happy, dont be afraid to stand up to the system.
There life outside school and those which keep this attitude up may be in for a shock when they join the 'Grown Up' world.