My DS10 just took the Iowa test for the first time. I was shocked to see how high his scores were. Does anyone have experience with the Iowa? If yes, would you say it is a pretty easy test?

Background: My son's last IQ testing put him between 140 and 150. He has an average processing speed. Normally he tests higher than most kids his age, but on the Iowa, his average grade was 9th (while he was in 4th). And, he scored 12+ in many areas. If it weren't for his math grades, which were around 6th grade level, he would have averaged being a typical 11th grader.