Well, Shamrock, all I can say is that some of the replies to your question don't exactly fit the description of ballroom dancers as well-mannered! :)
Seriously, though, ballroom dance is partner dancing. So it requires that the dancer always be aware of, responsive to, and considerate of the person they are dancing with. It's integral to the dance--you can't dance well without it. You literally have to cooperate on every step you take. So it encourages those qualitites.
It is also a pretty structured form of dance, especially as it is taught at the beginning levels. So there are specific patterns to learn and practice. That may lend itself to a more disciplined approach to learning and practicing than less structured dance forms.
Because it involves men and women dancing together (or boys and girls), it may encourage being on better behavior in class and while dancing. IME, both boys and girls tend to be on better behavior around one another than they are when with groups of only their own sex. Many other dance forms tend (unfortunately) to involve mostly girls.
Historically, many of the dances come from an era when they were part of a social context that emphasized etiquette and polite behavior. Some of that still exists in the ballroom dance world. For example, it's not unusual for the man to offer his arm to escort the lady on and off the dance floor. That's not something you see very often anymore outside a ballroom dance.
I was in my 40s when I started dancing, so I think my values and manners were pretty much set by then. So in my case, it may be that they influenced me to choose ballroom dance, rather than the other way around. That may be somewhat true of the high school dancers, too, but I'd guess that it does play a part in helping teenagers develop their value systems.
As another poster mentioned, the film "Mad Hot Ballroom" would be interesting for you to watch, as it clearly shows the effect that ballroom has on the kids involved.
Good luck in your research!