My elementary school age kids have had multiple teachers since kindergarten. Throughout the week, they go to separate classes for health and fitness (i.e. "PE")

My elementary school age kids have had multiple teachers since kindergarten. Throughout the week, they go to separate classes for health and fitness (i.e. "PE"), music, art, and the library. I don't think the transition to more separate specialized classes is a big part of it. It's mostly hormones and their brain powering up the social reasoning facilities and emotions without yet having the life experience to manage those feelings. Middle school is right around the time that your social status and reputation — your image in the eyes of your peers — becomes an important part of your sense of self and self worth. But that status and reputation is heavily influenced by everyone around you. It's like all of a sudden these kids all wake up with giant targets floating above their heads and rocks in their hands. By high school, they will have internalized some of the norms around how to not use the weapons they have — in large part because of their experiences now — but middle school is a time of experimentation and mistake-making, so there ends up being a lot of random collateral emotional damage.