How much does that have to do with its sanctuary status & high illegal immigration numbers.
California is home to 2.3 mil illegal immigrants, and 12.3% of all k-12 students in California are illegal immigrants. Other states with that high of an illegal immigrant student demographic are Arizona, Nevada, and Texas [1]. According to US News, Nevada is #49, Arizona #43, and Texas #41 in K12 public school rnakings by state. [2].
I'd assume that illegal immigrants are generally on the poor side. The children of illegal immigrants likely speak english as second language.
Generally speaking, kids who are ESL and kids who are poor score lower on standardized tests. Now imagine both poor & esl & having to deal with stress of being an illegal immigrant. These kids have everything going against them and are likely going to score worse. Put enough of these kids in a school, and you're going to bring down the education for the non illegal immigrants. You're only as good as who you surround yourself.
[1] http://www.pewhispanic.org/interactives/unauthorized-immigra...
[2] https://www.usnews.com/news/best-states/rankings/education