Allstar Sports is the go-to resort for youth sports and tour groups. If there are enough of them staying at any one time, they will branch out to the other Allstars and possibly Pop Century. Moderates can get them too, especially POR and CBR. I think POFQ's small size and CSR's status as a convention resort keeps them from getting as many. They generally do not stay at deluxes or DVC villa resorts.
Apparently some of the biggest times for youth groups to visit are during January and July when there are school breaks in Brazil, and during the Pop Warner football/cheerleading championships the first weekend in December. There can be other times too. Last weekend there were several groups on senior trips or school band trips (they usually wear matching tee shirts so you can tell what kind of trip they are on). At the end of this month there is a cheer/dance competition with competitors staying at the Allstars and CBR.
If you prefer to avoid large business convention type groups, avoid resorts with a convention center, especially those with a large convention center (CSR, Contemporary, Swan and Dolphin)