You have that backward.

The home-educated typically score 15 to 25 percentile points above public-school students on standardized academic achievement tests.

-- https://nheri.org/research-facts-on-homeschooling/#Academic

Site: National Homeschool Education Research Institute

First independent search result:

> The US-based NHERI describes itself as a leading research institute in the field of homeschooling. However, its neutrality is often questioned.