As Intelligence tests were originally devised by Alfred Binet to measure children's ability to succeed in school, it is perhaps not surprising that they do in fact predict school performance fairly well. The correlation between IQ scores and Grades is about .50. What people can know can be altered dramatically by altered teaching. However, the gap between high and low IQ outcomes remains. Years of Education.
Bright children stay at school longer.
All other things thrown in, test scores are the best single predictor of an individual's years of education. Mostly, this correlation appears because education is more rewarding for bright children. Negative correlates of IQ include Crime. By and large these correlations are modest and seem most easily to be explained as a combination of people having less to lose and being less able to find adaptive solutions to problems. In many ways, this debate is between two questions, rather than over which is the right answer to any single question.