PHOENIX — The share of tax dollars that actually winds up in Arizona classrooms slid again last year, to the lowest level in the 12 years the state has been monitoring.

A new report Friday from the Auditor General’s Office found just 54.2 percent of every dollar schools received was used for “classroom spending.” That is down a half a percentage point from 2011 and below the peak at 58.6 percent in 2004.

Classroom spending includes salaries and benefits for teachers, aides and coaches. It also covers supplies such as pencils and paper, athletics, and activities like band or choir.

But the 54.2 percent figure — or $4,051 per pupil — is only half of the story.

That percentage is applied to total spending per-pupil for all education costs, which was $7,475 in 2012, which is down for the third straight year.

For example, in 2009, total spending per student was $7,908, so the 56.9 percent that went into the classroom that year was $4,500 per student.

By comparison, the national average for classroom spending was in 2012 61.3 percent.

But there’s another side to the comparison. Auditor General Debra Davenport said part of the way Arizona keeps its instructional spending lower is by putting more children in each classroom.

She said in 2009, the most recent figures available, the national average class size was 15.3 students per teacher. Arizona’s figure that year was 17.1.

Despite the multiyear slide in percentage of dollars winding up in the classroom, Davenport said there are some bright spots in the report, at least statewide.

“The relatively low classroom dollar percentage was not the result of high administration costs, because Arizona districts allocated a smaller percentage of resources for administration than the national average,” she wrote in the report. The Arizona figure is 9.9 percent; nationally, school districts spend 10.7 percent of their budgets on administration, which includes superintendents, principals, business managers and other staff who do accounting, payroll, printing, human resource and tech services.

Within that category, however, size makes a difference.

Davenport said the largest school districts spent an average of just $632 per pupil on administrative costs due to the economies of scale. That increased to $800 for medium-large districts, $1,149 for medium districts and $1,519 for the smallest.

State lawmakers have tried for years to force consolidation of the state’s 227 school districts. So far, though, the best they have been able to do is offer financial carrots to districts willing to combine, with few takers.

Marty Shultz, who headed the 2009 School Redistricting Commission, said the problem is local resistance, including superintendents and school boards.

He said it was one thing for lawmakers to leave the issue to local control when the figures on classroom spending were closer to 60 percent. But now, with the continuing decline, Shultz said lawmakers are finally going to have to force the issue.

“It starves teacher salaries, it starves class sizes,” he said of the administrative costs. “It’s less money going to the front lines.”

State schools superintendent John Huppenthal said some of the high administrative costs for schools can be blamed on his agency’s “antiquated student data system.” He is pushing lawmakers to fund a new system that should cut total administrative spending statewide by $100 million, “which can be redirected back into the classroom.”

Davenport said what is making the noninstructional side of the equation so high in Arizona are the categories of plant operations — 12.4 percent here versus 9.5 percent nationally — and food service costs of 5.1 percent in Arizona compared to 3.8 percent nationally.

That issue of plant operations, she said, can be blamed in part on the fact that in the last five years Arizona schools added nearly 12 million square feet of building space, a 9.4 percent increase. Yet at the same time, the number of students attending dropped by 2 percent.

The result is that building capacity usage dropped from 81 percent in 2007 to 79 percent last year.

Davenport acknowledged some of the new construction was started before enrollment began to decline in 2009. But some of it, she said, was due to what has been the reluctance of some districts to reduce excess space.

She said, though, that attitude appears to be changing, with 26 fewer schools operating last year than the prior year.

“Although decisions to close buildings or schools can be difficult and painful, these decisions are important because school district funding is based primarily on the number of students enrolled at the district, not the number of schools or amount of square footage maintained,” Davenport said. She also said districts that completely close schools, including turning off utilities, can save more than those that continue to heat, cool and maintain closed buildings beyond minimum levels.

The increasing cost of food service, however, is something else.

Davenport said statewide, the cost of meals was only a nickel higher in 2012 than five years earlier.

But districts served 9.7 million more meals. She said that is not surprising “considering the state’s poverty rate increased from 17.7 percent to 25.1 percent during that time and the percentage of students eligible for free or reduced-price meals increased from 50 to 59 percent.”

Transportation costs also are up, making up 4.8 percent of districts’ operating dollars in 2012 compared with 4.3 percent in 2007.

Some of that, Davenport said, was an increase in the number of miles driven. That includes the effect of closing some schools and having to transport students farther.

But it also can be blamed in part to diesel fuel increasing from $3.04 a gallon in 2007 to $4.16 last year.