Native Spanish-speaking students are among the slowest to learn English of all immigrant children in California, taking nearly seven years to master the language, says the Legislative Analyst's Office in a report released Thursday.

Based on projections from current literacy tests, half of Spanish-speaking students become fluent in 6.7 years, compared to 3.6 years for native Mandarin speakers, who learned faster than all other groups, the study said.

Native Hmong speakers, a language spoken in Laos, took 7.4 years to master English, the longest of any group.

"I think the most important (point) is that it's taking kids longer to master English than is generally recognized," said Paul Warren, a senior policy analyst who wrote the report.

Warren said English proficiency rates were improving but more slowly than expected. His findings are based on results of a statewide English proficiency exam given over the last two years, so the report could not cite long-term trends.

Because students' progress varies dramatically depending on their age and native language, the report recommends that schools rethink the way they teach students. An array of services, including smaller classes and parent education programs, need to be offered, Warren said.

"What I'm hoping is that it will focus the system on really trying to understand what works for these kids," he said. "So often, the education system wants to find one silver bullet that works for all kids."

Some of the delays can be attributed to economic and cultural differences between types of immigrants, said Mike Dreebin, elementary vice president for United Teachers Los Angeles, which represents the district's teachers.

Students who immigrate from Mexico are more likely to be poor than students who come from farther away -- meaning their families are too busy working to place as much importance on education, he said.

"The poor people in Korea don't come to Los Angeles to better their lives. They can't afford to," Dreebin said.

About 42 percent of Los Angeles Unified's 740,000 students are English-language learners. While more than 90 percent of them speak Spanish, the other 10 percent speak some 85 languages.

LAUSD Board President Jose Huizar, who championed a resolution this week to research ways to improve Latino performance, said students can't wait six or seven years to master English.

"That's too long for them," he said. "They're losing out on advancing in other areas of their education."