The number of Fairfax County students who speak a foreign language at home is likely to surpass 50 percent of the school population for the first time this month, reflecting a surge of immigrant families in Northern Virginia, school officials said.

As total enrollment reached an all-time high of 181,500 students when school began Tuesday, Fairfax also saw a major increase in those who will need English language lessons. This year, about 31,500 students are projected to enroll in English for Speakers of Other Languages (ESOL), representing 17 percent of the total county student population and an increase of nearly one-third from last year.

Those numbers have profound financial implications for the school system, which spends $3,300 per student for ESOL lessons, county budget records show. With 7,652 new students in ESOL this year, that represents an additional $25.3 million.

Though federal law requires schools to offer English language lessons, federal funding cannot be used for such instruction, putting most of the costs in the hands of local governments.

“There is not an understanding in the community of how drastic this ESOL population growth is,” said Fairfax School Board member Ryan McElveen (At-Large). “When you have people that can’t speak the language and can’t learn other subjects because of that, it’s going to be a major hindrance to their education.”

McElveen said the board allocated funds to the fiscal 2013 budget to hire about 160 new ESOL teachers. Currently, 860 are on the faculty, officials said.

Immigrants have been fueling most of the population growth in Fairfax for years, census figures show. More than one in four county residents are foreign-born, and one-third speak a language other than English at home.

As a result, the number of students who need English instruction is the fastest-growing student population in the county, said Teddi Predaris, the county’s director of ESOL services. The same is true across the country and around the region.

In neighboring Arlington County, for example, ESOL students make up 16 percent of total enrollment. In Montgomery County, they account for 13.6 percent of all students; in the District, 10 percent.

More than 160 foreign languages are spoken in Fairfax County schools, the most prevalent being Spanish, Korean, Vietnamese, Arabic and Mandarin Chinese.

Many immigrant families are drawn to Fairfax, the nation’s second-most affluent county, with a median household income of more than $105,000, by the reputation of its public schools. That Fairfax “is a very international community, multilingual and multicultural, is an attraction” for foreign-language speaking families, Pedaris said.

While some ESOL students are immigrants themselves, the majority were born in the United States to immigrant parents. But not all the children who live in homes where English is a second language need additional English instruction, Predaris said. She said many children are bilingual and are already proficient English speakers.

At Lynbrook Elementary School, principal Mary McNamee said between 80 and 90 percent of the 560 students — pre-kindergarten through sixth grade — speak foreign languages at home. Officials said the school in Springfield has one of the highest concentrations of ESOL elementary students in the county, with more than 75 percent of the school’s students receiving such instruction. Bailey’s Elementary School for the Arts and Sciences in Falls Church, with an enrollment of 1,300, has the most ESOL students, with more than 660.

In Rose Martin’s fifth-grade class at Lynbrook, 16 of the 18 students have parents who were born outside the United States, including 14 from Central and South America and one each from Lebanon and Thailand. Three of the students were born outside the country.

Martin said teaching such a diverse group can present challenges. She said immigrant parents sometimes don’t involve themselves in the education of their children, in part because they have a deep respect for teachers. Martin, the daughter of Mexican immigrants who worked at tomato farms and picked cotton in California, is active in an effort at Lynbrook to forge relationships with students’ families to help get parents more engaged at school.

“In America, it’s like, ‘What are you doing with my kids?’” said Martin, a six-year veteran of Fairfax schools. “In other cultures, teaching is such a respected profession. They wouldn’t dare do that.”

Mary Yao, an ESOL instructor in Fairfax County for more than 20 years, is one of four teachers at Lynbrook who lead small group classes for students struggling to learn English.

She said the ESOL teachers see as many as 200 students a year for more individualized English instruction outside the classroom.

Yao, who was born in China, said many of the children she meets have weak English vocabularies as well as limited knowledge of their native languages.

“They live in a community concentrated in their own culture, and it’s hard to pick up English,” Yao said.

The county’s ESOL program began in 1975 with 275 students, most of them refugees from Vietnam. In 2008, the program reached 21,751 students and has since increased 44 percent.

McElveen said that the growth of students needing English lessons will be an issue he intends to address this year.

“I believe it’s great for our school system to have this dynamic population, because [the students] learn about people’s heritages and languages,” McElveen said. “The only problem is that it’s going to get more extensive over time.”