Nearly 60 percent of Cobb schools are underenrolled, a trend that may intensify as the district's growth boom slows for the first time in several decades.

Yet, there are more than 100 portable classrooms on campuses that aren't full by the state's counting.

Bob Andres/AJC (ENLARGE)

Students fill a hall at Kell High School in Marietta. Cobb County officials dispute data that indicate Cobb schools are below capacity.

PORTABLES IN COBB SCHOOLS THAT ARE UNDER CAPACITY Occupancy Rate Portables Baker ES 80 % 2 Brumby ES 95 % 9 Campbell MS 76 % 7 Campbell HS 95 % 12 East Cobb MS 89 % 7 Garrison Mill ES 88 % 2 Griffin MS 94 % 4 Keheley ES 88 % 2 Kennesaw ES 93 % 3 Lassiter HS 94 % 2 McEachern HS 98 % 13 Nicholson ES 85 % 3 Norton Park ES 95 % 2 Osborne HS 73 % 4 Russell ES 68 % 2 Rocky Mtn. ES 98 % 3 Skyview ES 97 % 11 Smitha MS 91 % 4 Sprayberry HS 81 % 1 Tapp MS 81 % 1 Wheeler HS 96 % 7 TOTAL PORTABLES: 101 Source: Cobb County School District Comprehensive Annual Financial Report 2007 RELATED: Cobb news Georgia and metro news

The simple math is misleading, Cobb school officials say.

The state tally doesn't take into account programs for special education, gifted, at-risk students and other special programs, some of them mandated by state or federal law, that don't fill classrooms to capacity.

The majority of vacant classroom seats are in elementary schools, according to the district's latest accounting.

An analysis of data of school capacity and enrollment in the district's Comprehensive Annual Financial Report 2007 released this month shows 63 of the district's 107 schools below capacity and 44 over capacity.

Across the county, there were 3,555 empty classroom seats at the end of the 2007 school year, according to the report.

At the same time, 475 portable classrooms are sitting on campuses from Acworth to Mableton, not all of them at schools considered overcrowded by the state.

Campbell Middle, for instance, has seven trailer classrooms even though the south county school was enrolled at only 76 percent of capacity. Campbell High rated a dozen portables even though the building's classroom space was 95 percent of capacity, according to the numbers.

School officials warn against falling into the numbers trap since classrooms don't hold neat packages of students in bundles of 30.

A classroom for 30, for instance, may hold only four autistic students, or 10 students learning English. A few advanced calculus students may take up a room intended for 30 students, said Superintendent Fred Sanderson. The district is increasingly encouraging students to take college-level Advanced Placement courses.

Sanderson explained that the state calculates its numbers as if the capacity of every classroom in a building is occupied by a maximum number of students.

"That's not the reality of what we deal with," the superintendent said. "In Cobb, because of special programs, special education classes or AP classes, for instance, you may have four autistic students taking up an entire classroom. It's required by law."

"You may have five or six units [classrooms] in a building that could have a total enrollment of 30 kids, but don't because of special programs and guaranteed course offerings," Sanderson said. "That's why you wind up with schools that by the state capacity formula are underenrolled."

Some schools have large special education programs or English for Speakers of Other Languages classes with five, six or 10 kids, Sanderson noted.

"Obviously, we would not put portables at schools if there were room inside the building," he said. "Our classrooms are being utilized very well."

In some cases, expanding middle school science labs nibbled away at classroom space.

Another example is Osborne High. By the state Department of Education's reckoning, Osborne is only 73 percent enrolled, yet it had four portable classrooms last year. "There's not a classroom that's not being utilized well there," Sanderson said. "They're making full use of the facility."

At Russell Elementary, with 311 open seats inside the building, there are two portables on campus. One of them is used as a mock space shuttle.

Associate Superintendent Gordon Pritz said underenrolled doesn't mean underutilized. "In some older schools where we've converted classrooms, they can't hold that many kids," Pritz said.

As the county gears up for a third special school sales tax referendum in September, members of the school system's citizen watchdog Facilities and Technology Committee are personally visiting each school to see for themselves space utilization, construction and technology needs.

Portable classrooms aside, the state enrollment and capacity numbers do point up enrollment bumps.

There are nearly twice the number of significantly underenrolled (80 percent capacity or lower) schools than significantly overenrolled (120 percent capacity or over) ones.

Among the bulging schoolhouses are Harrison and Kennesaw high schools, which will be relieved next year with the opening of Allatoona High, currently under construction. Significantly crowded Lewis, Vaughan, Ford and Bullard elementary schools will likewise see numbers drop with the opening of the new Picketts Mill Elementary, also in August, said Dennis Campbell, school system planning director.

New Hillgrove High is yet to fill as students opt to stay at their current high schools. The new facility will take about three years to fill, Campbell said. Once Allatoona High opens, the two most crowded high schools will be North Cobb and South Cobb, he said. Board members are discussing the possibility of building separate ninth-grade centers on those campuses.

As for the growth bubble deflating, Campbell said that 10 years ago, "you wouldn't think we'd see so many [schools] under capacity. Cobb is now at the point where the district's building program is beginning to catch up with population growth.

"Now the problem becomes old schools that are not sufficient," he said. Mableton and East Side elementary schools, both built in the early 1950s, are two examples, Campbell said.