This is the second annual report on a class-size cutting policy in the state - analysing results from 1998-99 - and again researchers are linking improvements to children learning in less crowded groups.
The latest study - the second of four planned annual reports - shows that the improvements among eight-year-olds are made regardless of any other factors, such as ethnicity and social background.
But the degree of improvement was seen to be small - with the progress on class sizes countered by the difficulty in recruiting well-qualified staff to teach the extra classes generated by cutting class sizes.
Last year's report found there had been rapid improvement among eight-year-olds in the first year of class-size cutting, but that deprived pupils were the least likely to benefit.
Class size cuts in the UK
Researchers say that in the period examined, California's class-cutting policy was almost fully implemented in the first three years of primary school - with 92% of schools having infant classes of less than 20 pupils.
California's statewide project to reduce class sizes is mirrored by a similar national scheme in the United Kingdom to set a maximum class size of 30 for infants.
The UK government has promised that all five, six and seven-year-olds will be taught in classes with no more than 30 pupils - with the prospect that class-size limits could be set for older pupils.
Close attention will also be paid to the ongoing findings of the California class size research by the United States education department - as President Clinton and Education Secretary Richard Riley have also promoted the cause of smaller classes.
Vice-President Al Gore, in his campaign for the presidency, has presented class-size reduction as an important element of his proposed education policy.