I worked at a summer camp for a few years. It's split into "sessions" for different age groups by grade level: 4/5, 6/7, 8/9, 10/11/12. It was staggering how bad the bullying was in the 8/9th grade session compared to all other grade levels. The rest of the groups barely ever had any issues at all other than isolated, subjective cases, while the 8th/9th grade sessions would almost all have HUGE drama with kids (boys and girls both) changing friend groups, kicking kids out of their group, and joining new groups, all within the span of one week at camp. More drama than a reality TV show, it was nuts.
I agree about eliminating middle school / junior high. It's too narrow a band of ages, they aren't exposed to enough more mature kids to learn how to act, or much less mature kids to show them a reflection of what they used to be like.
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).