Some critics argue that while home-educated students generally do extremely well on standardized tests‏¹⁴, such students are a self-selected group whose parents care strongly about their education and would also do well in a conventional school environment.

Some opponents argue that parents with little training in education are less effective in teaching. However, some studies do indicate that parental income and education level affect home-educated student performance on standardized tests very little.

http://en.wikipedia.org/wiki/Homeschooling