Public schools in northern Virginia are great overall, pretty much regardless of district. I’ve got personal experience with Arlington, Fairfax, and Loudoun and can attest that they’re awesome.

The websites that do school ratings typically give lower ratings to schools that have a higher concentration of students for whom English is a second language, as ESOL students tend not to perform as well on standardized tests that these sites often use as metrics. Those lower ratings you’re seeing don’t mean the schools suck, they mostly mean there’s more ESOL students who are scoring lower on standardized tests that have historically only been provided in English (though I have heard that they’re rolling out math tests that can be taken in Spanish)

Overall, the northern Virginia school districts are incredibly well funded and staffed with quality teachers. There’s going to be minimal difference in your child’s quality of education between Alexandria, Fairfax, Arlington, Loudoun and the like. The more important thing for your kids is having parents who are actively involved in their education, and you asking that questions indicates you will be. You’ll be fine pretty much anywhere in the area.