I have been finishing up my dissertation which deals with K-12 online learning in California. Official enrollment figures are only available for full time students in online charter schools. When looking at just online charter schools in California, the number of online students has increased more than 40% each year. No doubt there are many more. See more info here.
Secondly, I created a timeline/history of online learning in California set in a national context. The idea of distance education began in 1967 by German, Otto Peters. Various laws and policies in California have driven K-12 online learning in California. See the timeline here.