Anecdotally, as a parent, I've found the opposite to be more true. We have several schools that are k-8 in my city, but the majority split 6-8 into middle school. Talking to parents of the k-8 schools, bullying is almost non existent. Not true in the traditional middle school. Some kids are more or less mature than others as they mature, and it helps to have options for friends. Also, kids weren't split into single year age bands for all of the existence of humanity and then the past hundred years or so we suddenly split them up.