I think that a child who didn't want to work too hard might do poorly on the pretest to avoid getting harder words. My ds used to go to a school that used Words Their Way, where they pretested the class at the beginning of the school year and had spelling groups after that. Each week, the words would follow a pattern that the kids would learn. After the early elementary years, some kids were finished with the program, so they worked on vocabulary instead with WordMasters,