Are there studies on whether bullying is higher in lightly supervised versus moderately supervised groups? Or mixed-age versus single-age groups?

Scouting is lightly-supervised mixed-age groups. If an older kid bullied a younger kid, that resulted in adults reading them the riot act. But if a younger kid bullied a younger kid, the two sort of wound up sorting it out until someone threw a punch or pissed off an older kid. (For being annoying.) That second dynamic was, to my memory, unique to mixed-age groups.