The 6.3 percent increase in the Arlington County schools budget will help provide more classroom space and expanded programs for the fast-growing student population, officials said.

The School Board explained the reasoning behind its approved $469.8 million fiscal 2012 budget at a joint work session Monday night. The board’s budget is $1.6 million more than what County Manager Barbara Donnellan proposed for the system.

School officials estimate that the system, which has 21,000 students, will be at capacity within two years. Enrollment is projected to rise by 5,700 students by fiscal 2016.

“We are proud of the fact we spend more per pupil” than neighboring jurisdictions, said Christopher Zimmerman (D), County Board chairman, who said the projections were no surprise. “That is part of the success of what we are doing.”

The per-pupil cost this year is about $18,080, compared with $17,300 in fiscal 2011.

The School Board’s budget includes a $12.3 million increase to handle rising enrollment and the system’s capacity limitations, said Abby Raphael, vice chairman of the School Board.

The funding tied to enrollment includes $1.2 million to add as many as 30 relocatable classrooms to the system’s 62 units. It also provides increases in transportation costs, conversion of computer labs to general classroom space and $700,000 to plan and design additions “so we can have the best estimates possible” for the capital project planning process with the county, which begins in the fall, Raphael said.

The School Board plans to dedicate $8.6 million in estimated additional county revenue at the close of fiscal 2011 toward that design-and-build process, Raphael said.

“We are hoping to do as much as we can without bond funding,” said Libby Garvey, School Board president. “And we have to start now.”

The budget also includes larger high school classes, expanded day programs for middle school students, and evaluations of classroom and special needs programs, including those for autism.

The number of autistic students attending Arlington schools has increased threefold since 2003, to about 375 children this year, which follows national trends, officials said.

Federal guidelines require a teacher and two aides per six autistic students and the same number of adults for up to 10 children with Asperger’s syndrome, a high-performing form of autism, a rate that is much “more intensive” than requirements for a special education classroom, said County Board member Mary H. Hynes (D).

Barbara A. Favola (D), County Board member, said she was interested in a “comparison of what the feds are funding compared to what we have to make up” when it came to new autism programs.

Special needs programs and those to encourage students to earn specialized diplomas, among others, are what are driving the cost per pupil higher this year, Garvey said. State test results show that Arlington students are improving and narrowing the achievement gap between races and ethnicities, but those gaps still exist.