All you can do is your little bit to challenge the straight-line thinking. Good on ya for trying.
Considering the age of the participants and their evident inability to consider ideas rationally, I'd say that they're most likely parroting the opinions they hear from their parents, peers, and media. And as you rightly pointed out, much of that behavior stems from acting up to "impress" their peers. Messenger is reasonably anonymous, so they probably felt like they could get away with being idiots.
Hope that the penny drops sometime. Maybe they will be more receptive when they start reading books and get taught something about history/current events and life in general. When is history taught in England's curriculum? I know that I didn't get taught 20th century history in a systematic manner until I hit grade 12.
If you're interested in negotiation, mediation and debate, you could do worse than hang out in CE - all those skills will get a workout, guaranteed.
