Maybe your school just works differently. I'm in my 6th year at mine, and the cliques here have been utterly impenetrable since day one. Even tried joining a club that matched my interests, only to find that the existing club members had this "circle the wagon trains" thing going on, to the point of being borderline hostile to an incomer like me.
My experience with the working world has been much the same, only there you have to navigate the minefield of office politics, where one wrong step can get you reported or fired. I will concede that high school is worse, but what comes after still isn't exactly love and skittles.