If you find that the IQ of 5-year-olds predicts many future outcomes, that will be a badly needed reminder that schools CANNOT make everyone good students.
You'd need to adjust for significant confounding factors, though. The heritability of IQ is pretty low for 5yos, so much of what you'd be testing for is socioeconomic status.
A good school environment can do quite a bit to overcome SES - Head Start gives kids a pretty good IQ boost (which fades over time, although it's not clear if that's a heredity issue or a decreasing quality of school environment issue).
