Superintendent Jack Dale proposed a record $2.4 billion budget for Fairfax County Public Schools on Thursday, citing the booming enrollment of students with challenges.

Dale is asking the county to put up $1.75 billion, an 8.4 percent increase over fiscal 2012, when the Board of Supervisors gave the schools a flat transfer after cutting their budget the year before.

"I'm required to put forth a budget that meets the functions of the school system, and I don't think I've even done that," said Dale, defending the relatively large request. "I'm undercutting the needs of the school system."

Most of Dale's budget, or $2.1 billion, goes to costs associated with instruction. The majority of 720 new proposed positions address the school system's whopping enrollment growth and demographic shifts.

The largest school system in Virginia and 11th-largest in the nation, FCPS is expecting an increase of more than 3,900 students next year, capping off an influx of 15,000 students in the last five years and reaching an enrollment of 181,608 students. By next school year, the number of students who require English for Speakers of Other Languages is expected to have grown 48.3 percent since the 2007-2008 school year. The number of students who qualify for free or reduced-price meals -- the district's indicator of poverty -- will have grown 35.4 percent.

Class sizes for ESOL and low-income students are typically smaller, requiring more teachers, Dale said.

Mary Kay Downes, an English and journalism teacher at Chantilly High School, said she was "constantly amazed" by how well those students assimilate in the schools and bring "such a richness and diversity to our classes."

"And this kind of success costs money," Downes said.

Dale's budget also includes $36.6 million for a 2 percent cost-of-living increase and $42 million for pay raises, and accounts for a higher contribution to employee retirement funds. It also provides a pay raise for custodians, and provides benefits for the first time to parent liaisons and multilingual interpreters.

The school board has scheduled public hearings on the budget for Jan. 30 and Jan. 31.