As with any other demographic, there's a huge variety. When I was in secondary school (pretty decent, inner city state school with a huge mix of demographics), a teacher started a debate club and we ended up doing county wide competitions where every other school involved was private.
Now, a lot of the people I'd made friends with at my own school were very academic and came from comfortably middle class families but this was like meeting another breed entirely, the boys especially. It was the sheer confidence, rather than academic ability, which stood out. Boys as young as thirteen who'd stand up and talk utter bollocks (which was inevitable, with some of the points you had to defend) with the complete conviction that everyone in the room wanted to listen to them speak. There didn't appear to be any of the self consciousness that I felt that you were taking up people's time and attention, and feeling under pressure to make it worthwhile for them.
They seemed mostly nice people and many were very intelligent, but I doubt that those who weren't would ever be called out on it.