I like your idea of "mixed up brain wiring" - this is probably the best descriptor I've heard!!
The school you are looking at may be a bit smaller than my son's school. Some smaller schools group the by skill level and profile rather than grade. My son's school was large enough to group kids by both grade, skill and profile as there were 63 students in his grade. When there is less "critical mass", the school has to be more flexible in how they group. There is another well known and respected LD school in my state that is relatively small in the elementary and MS grades (about 20 kids per grade), and like the one you are looking at, sometimes group kids together from different "grades".
