Final answer:
The most supportive idea for an essay against playing football in middle school is highlighting the danger of the sport and its potential for causing serious injuries to young athletes.
Step-by-step explanation:
The idea that best supports an argumentative essay against playing football in middle school is the assertion that football is a dangerous sport. Concerns arise from the high risks associated with football, such as the potential for concussions and other serious injuries due to the physical nature of the game. Young athletes may be particularly vulnerable to these injuries, which can have long-term effects on their health and well-being. In contrast, the popularity of football, its audience size, and its origin are less relevant when arguing against the sport's safety at the middle school level. Addressing the safety issues directly correlates with the aim of the essay to advocate for the prevention of injuries among young students.