These groups travel year round lately, but you have 100% chance to hit one from mid June-Mid august (July being the hell on earth). Also from Mid Nov-Mid February (December (after the 15th and January being the peak)
Also the week that hold Oct 12 will have a significant improve in the amount of brazilian groups.
All those periods above are school out periods.
I´ll be in the World from Dec, 10- Dec 17 2011 and I´ll let you know how it was...
Hint: Most groups follow a schedule so it´s a great idea ask the group guide (one with the flag and talk-about radios) about their plans for the day (and even for the week... Most brazilians love to chat and I think they will share this info with pleasure.
Also If you feel a group is behaving in such a way that is inappropriate or even disturbing you (or your family), please talk to the tour guide. He/She should be able to handle the situation quiclky and let him/she know that you ARE going to complain with a WDW CM if nothing is done.
My 2 cents as brazilian is: they should behave or should be kicked out. PERIOD.
Oh and BTW... Most of brazilian are nice, educated and easy going people

But teens...
Thanks for your reply! Also thanks for the others who answered my questions as well.
I sort of know the South American teen tour groups because I'm originally from Argentina (I live in Spain though). I went to WDW in 1996 in one of these groups myself, but we were a very small group (approx. 18 kids with 2 chaperones). We were assigned free time every day to enjoy the parks ourselves but I preferred to be at the parks by myself alone so I stayed away from my own group

. My group was quite quiet and well-behaved but we liked chatting (and we're loud when we're happy or having a good time just as brazilians) and if we felt like it, we would sing a song (not too loud) to make queing less boring. Of course we would have never thought that was considered an impolite thing in the U.S. no travel book tells you that singing in public can be rude to others

. But, if we inconvenienced anyone nobody let us know about it.
As I said I was more worried about Pop Warner week. I was choosing first week of December because summer holidays for southamerican kids don't usually start until the second week of December so I was trying to stay away from the tour groups as well... but now I see that Pop Warner is the first week of December so I'm kind of bummed. Also now I see that these "quinceañeras groups" stay at the moderate resorts as well?! Back in 1996 they would only go to the value resorts ...
Even when I was trying to avoid them I said that I could stand the tour groups because if I need to I can scold the kids in their own language XD so I'm not that worried about them. Still, we were hoping to choose a nice quiet hotel and the moderate ones aren't exactly cheap ;(. But, if Pop Warner isn't THAT bad then I'm relieved.
I'm with you, tour group kids should behave or they should be kicked out. Never be afraid to talk to the kids who are misbehaving. They might not understand what you say but they will definitely "get" the idea that they're being scolded for bad behaviour. Also, speak to their groups leaders and make sure you leave a complaint at Guest Services or at your hotel of choice if they disturb you in any way.
If they're doing something out of place or disturbing to you let them know. They might even try to start a conversation with you and start some cultural exchange

(or at least they'll stop doing whatever they were doing)
Also, if you get the website or the
travel agency they're from you can always send them a complaint. the travel agency will care very little but, if the experience with these tours is being improved in any way then Disney (or these travel agencies) has to be doing something right .
Let's hope the experience for everyone keeps improving more and more over the years. Seeing that they get a super greeter sounds like a very good idea!
Have a nice week everyone!