With only five days of school left, advanced-math fifth graders at Renaissance Academy-Edison Charter School considered a new concept: how to compute the surface area of a juice box.
"I know you're up for the challenge," said teacher Shari Benowitz, 31.
The children, wearing red polos and khakis, huddled in small groups to strategize as Benowitz leaned over them and offered encouragement.
Such rigorous curriculum and individual attention, administrators say, have boosted the Chester County school's standardized math and reading scores substantially since it opened in 2000. Last month, the Center for Education Reform recognized that improvement by naming Renaissance a national charter of the year.
"It was progress over time," said Jeanne Allen, president of the Washington advocacy group, which supports charters. "There was just no question they were exceptional."
Once found almost exclusively in urban centers with dismal academic options, charters such as Renaissance - located in a bucolic corner of the solid-performing Phoenixville Area School District - have become increasingly common in the suburbs.
Chester County in particular is a charter hotbed. Outside of Philadelphia, where 56 charters serve 27,500 students, the county leads the region with an enrollment of nearly 3,600 in seven schools. Graystone Academy in Coatesville focuses on a classical education; Sankofa Academy in West Chester on African American culture. Renaissance has a college-prep curriculum with extra helpings of dance and drama.
Only the city of Camden, where about 2,300 students attend a half-dozen charters, and Delaware County, with about 2,600 at three schools in the troubled Chester Upland district, come close.
The reasons for the suburban boom are as varied as the schools themselves.
Parents value choice, even when districts are strong, education experts say. Some are drawn to a special curriculum or an extended school day.
"People just go to the program that works for them," said Timothy Daniels, executive director of the Pennsylvania Coalition of Charter Schools in West Chester.
In fast-growing Chester County - where the school-age population has ballooned 28 percent since 1990, according to the Pennsylvania State Data Center - the popularity of charters may have another explanation, experts say.
These self-managed public schools, which are approved by their local districts, typically offer the small enrollment associated with private institutions, but without tuition.
In 2002, five years after Pennsylvania adopted legislation allowing for the creation of charters, the Avon Grove School District opened a consolidated intermediate school for 1,600 children. Though limited to grades three to six, it was the largest elementary school in the state.
The same year, parents advocating smaller neighborhood schools got approval for Avon Grove Charter School in West Grove.
"It was all about size," said Kevin Brady, chief academic officer at the charter, where 1,100 students attend kindergarten through high school.
"Frankly," said Bill Winters, chief executive officer of Collegium Charter School in Exton, which draws a third of its 900 students from the West Chester district, "if we were to close, the district would have to build another elementary school. . . . We're an escape valve."
The nation's 4,000 charters dot suburbs in increasing numbers, especially in high-growth communities in California, Florida and Pennsylvania, Allen said. Bucks and Delaware Counties have three each; Montgomery has one. (Additional cyber charters based in Chester and Montgomery Counties attract online learners who can reside anywhere in Pennsylvania.)
Last month, for the first time, Allen's group honored 53 charters. About a third, including Renaissance, were not in urban centers.
One of the top performers managed by for-profit Edison Schools, Renaissance is on the grounds of Valley Forge Christian College. The Phoenixville school board approved it unanimously, but with trepidation, said charter liaison Mary Parris, a 16-year board member.
Many members were wary of its toll on district finances and worried it could lure away top scholars. "We were fearful of the unknown," Parris said.
During the last academic year, Renaissance's K-12 program attracted 267 students from the Phoenixville district, which enrolls about 3,300. About 625 more came from Owen J. Roberts, Norristown, and 15 other districts.
For each Renaissance student, the home district redirects roughly 80 percent of its per-pupil expenditure to the school and receives partial reimbursement from the state. Phoenixville funds about $3 million of the charter's $10 million annual budget.
Renaissance, Parris said, has "helped us with some overcrowding issues. I don't think we're feeling the pinch as bad as we might have."
More important, she said, it expanded education choices.
"Most people are satisfied, or more than satisfied, with Phoenixville public schools," Parris said. "But there are people looking for something different."
Cathy Greenhow of Paoli has six children. The older three went to the heavy-hitting Great Valley School District. But she felt students considered neither gifted nor remedial - what she calls "in-between kids" - could fall through the cracks there.
"I wanted a private school," she said, "but I couldn't afford a private school."
In 2001, she moved her younger children to Renaissance. She loves its size and core values. Daughter Amber, 18, a graduate, now studies at Gwynedd-Mercy College.
Megan Gorman, of Collegeville, came to Renaissance for sixth grade after struggles in the Methacton Area School District. "I [got] a lot more attention," said Gorman, who just graduated as class salutatorian and will attend Millersburg University.
When Andi Fanelli McGunnigle's son ended kindergarten in the Phoenixville district not knowing how to read, his mother wanted an alternative. That led the former finance executive and seven other parents, none with education backgrounds, to brainstorm. A year later, their kitchen-table scribbles led to Renaissance Academy.
From day one, it attracted students with a range of abilities from numerous districts, more than anyone expected. The last school year's wait list was 241; the school plans to expand to 925 students in September.
"It was amazing to us," said McGunnigle, the school's strategic project coordinator. "I guess there was a need."
Inside the airy building, where banners extol values such as integrity and justice, students follow a curriculum that, in McGunnigle's words, "treats all children as gifted."
Spanish lessons and grouping students by ability starts early. Next year, Renaissance will stage a full production of The Nutcracker. Teachers stay with students for two years to foster consistency, and high schoolers earn dual credit for college courses, some taught on-site by professors.
Chief academic officer Gina Guarino Buli talks about parents as customers, faculty as salespeople, and Renaissance as a business.
In her office, a white board is covered with the school's 2006 PSSA scores, the most recent available for the standardized state tests. From 2001 to 2006, Renaissance's overall percent of students at "proficient" or "advanced" levels rose 38 points in math and 32 points in reading.
Students performed above the state average and close to Phoenixville's level in 2006, but the percent of fifth graders proficient or better in reading dropped 11 points from the year before. Eleventh graders showed losses in math and reading as well, though third and eighth graders gained.
"We spend a lot of time looking, talking and analyzing what's behind those numbers," Buli said. Improvement depends on individual performance, something a charter's flexibility can address, she said.
On a June day, an inquisitive Renaissance fourth grader wondered how the number of Pennsylvania farmers could dwindle since the 1800s when agricultural production has increased.
David Cosme, 29, who teaches grades three and four, put the query to the class. That led to talk of technology - and a math lesson.
Such sessions, Cosme said, stretch top students and clarify concepts for others.
"It's a challenge," he said of the range of abilities, "but we get it done."
Contact staff writer Lini S. Kadaba at 610-701-7624 or Lkadaba@phillynews.com.