They have not.
Over the past 18 months, the number of children with limited English-language skills who have improved their scores on the nationally recognized Stanford 9 test has doubled, according to the California Board of Education.
"There's a lot to be said for immersion," said Annette Kessler, principal of the L.B. Weemes Elementary School in Los Angeles.
Scores up despite worries
Scores on standardized tests at L.B. Weemes are up six to 14 points in every subject and at every grade level, said Kessler, who taught bilingual classes for 10 years and found the program wanting.
"There was no continuity in how the program was delivered," she said.
Other educators, however, say that crediting the sink-or-swim approach for higher test scores is too simplistic.
They note that since the English-only policy was adopted -- as the result of a referendum called Proposition 227 -- California also has cut class sizes in its lower grades, and educators have shifted back to the basics.
But English-only proponent Ron Unz, a Silicon Valley entrepreneur who almost single-handedly financed the referendum, is behind a similar November ballot initiative in Arizona.
"When you look at those countries around the world that simply stick to their traditional subject matter in their schools, they do a lot better," said Unz.
Critics unconvinced
Not all teachers are convinced.
"They can read phonetically, but they still don't have the language development that they need," said second grade teacher Mayra Urbina of bilingual students.
California's Proposition 227 has a provision that allows parents to obtain waivers, allowing them to transfer their children into a bilingual program. To do so, they must show that their children would suffer educational or emotional problems in English-only classrooms.
This school year, the California Department of Education has granted 188,000 such waivers.
That hasn't been the case at Weemes Elementary, where Kessler says not a child has been taken out of English-only classes.
CNN correspondent Jennifer Auther contributed to this report.