Totally agree, which is why I think junior high is often the worst when it comes to bullying. In my school district elementary schools (up to 6th grade) had about 100 kids per class, and they all fed into one massive junior high that had about 1500 kids total in 7th and 8th grades. Plus, that is the time when kids are going through puberty (often at vastly different rates and times), trying to figure out their own bodies and hormones.

Your description of "The bullies feel vulnerable and are looking to prove themselves, and the bullied are vulnerable, having not yet had any chance to form social support groups." was spot on in my experience.

Frankly, I don't understand the purpose of junior high at all, seems like it would make much more sense to just have elementary schools go up to, say, 8th grade, then go to a high school (lots of private schools already do this).