IIRC most pediatricians advise against caffeine for children under 12. But explaining stuff like caffeine might be hard for children, so they simply just ban them from drinking coffee (by saying it's an adult drink, it's toxic for children, etc).
That's why one of their classmate says that them drinking coffee is worse than soda, since it has more caffeine.
it's not about the coffee,
there is a (weird? and strict?) rule in Japan (I don't know if it is written rule or not) that elementary school students are not allowed to take detours on the way to or from school, which includes buying snacks,
so the girl snitched about that