" the shocking fact that in the LA County school district almost half of the children scored below average
on a reading test."

WWh noted :Insert two words writer forgot and it makes sense.

the shocking fact that in the LA County school district almost half of the children scored below the national average on a reading test.

And even more sense if you say "...on a reading test in standard English." Many youngsters in LA County are children of immigrants and are probably fluent in a language (or two) other than English/American. Sadly, more than can be said for most of children in many other US school districts.