Arlington County Superintendent Patrick K. Murphy proposed a $470 million budget on Thursday that would give employees their first raises in two years but would increase average class sizes in the district.

Like most other Northern Virginia school divisions, Arlington is working to reconcile a tight budget with substantial projected enrollment increases. Murphy’s budget proposes a 6 percent increase from this year’s, as the state’s 14th-largest school district prepares to absorb an additional 1,000 students in the 2011-12 school year.

“We had to make some tough choices again this year,” Murphy wrote to School Board members in the introduction to his budget report. Those choices are likely to become more difficult in years to come. Enrollment is projected to grow from about 21,000 to more than 33,000 in 2016.

The Arlington school system spends more per pupil than any other school district in the Washington area, and it has some of the region’s smallest average class sizes. Murphy’s budget would increase per-pupil spending from $17,322 to $18,115, in part to fund additional instructional time at four schools. But Murphy also proposed increasing average class size in grades 9 to 12 by one student, from 19.2 students to 20.2.

Prince William County, which is spending $9,577 per student this fiscal year, has an average high school class size of 30, according to the Washington Area Boards of Education. Fairfax County is spending $12,597 per student and has an average high school class size of 25.3.

Employees will receive either a step increase or, if ineligible for a step increase, a $1,000 pay increase. But salaries will not include a cost-of-living adjustment. Employees have not received such an adjustment for two years, or since the 2009-10 school year.

Murphy said he expects the school district’s David M. Brown Planetarium to remain open, thanks to fundraising efforts of area residents. During last year’s budget struggles, Murphy said the planetarium would be closed unless substantial amounts could be raised through private donations for capital improvements.

His budget proposal this year is markedly less dire, even as the district works to make up for the loss of more than $3 million in expiring federal stimulus funds. Murphy is proposing to spend $27 million more than was budgeted for the current fiscal year. The increase includes funding for 95 additional positions, two additional preschool classes and expanded after-school programs.

“This is money that gets right into the classroom, that acknowledges that teachers are the ones who make a difference in student learning,” he said.

The county School Board is scheduled vote on Murphy’s proposal March 24, and the County Board is scheduled to act on a final county budget April 28.