I don't think people are actively being racist, but people of the same cultures/religions/backgrounds tend to spend time together because they have more in common. I'm not in the slightest bit racist, but the majority of my friends are white. In the same way I don't think the large group of muslims in my school are racist towards non-muslims, they just get on well with eachother because they have a lot in common. Noone has to stick to these groups though, and other than the few actually racist people out there, I doubt anyone's looking at the colour of people's skin when making friends.