Lower high school.
In elementary school I was so young that I wasn't very aware of myself in some ways, which helped me, because when I did become more aware, I had already gotten to know the other children more. Even at that age I seldom knew what to say, but we could play and that helped a great deal. In junior high the majority of the class was from my old elementary school so I knew most of them already and were comfortable with them, and the rest I got to know while there. We were still young enough to be somewhat playful and kidding around a lot, and that helped too.
In high school I didn't know anyone. And we were too old to play (well, there were some water war games, but that came later when we knew each other better), so it was much harder to get to know anyone. Having little to say + No common activity + No common back ground = A hard time getting to know people. The first two times I didn't at all, because I quit after about 2 months (and I skipped a lot of school in that time).