Third-grader Valeria Sanchez sat toward the back of her science class Tuesday, listening as her classmates asked questions about an upcoming assignment. She had a quarter-sized sticker on her cheek, a reward for getting a perfect score on a math test.

Math is Valeria's favorite subject. She recently learned how to use tape diagrams to draw out the problems using dots and lines: "It helps me find math answers easier," she said.

To her teachers and classmates at the KIPP Connect charter school, Valeria is known as part of the Class of 2025. Students are designated by these far-off years to emphasize college preparedness as early as pre-K. Teachers and school leaders emphasize that this isn't just the year these students are expected to graduate high school - it's the year they'll start college.

Such high expectations have helped the Houston-founded charter chain achieve continued gains in student achievement despite some minor speed bumps during massive growth, according to a comprehensive new study.

The final piece of an eight-year, $10 million research project on KIPP, whose mission is to help low-income students graduate from college, found that campuses across the nation continued to increase test scores at every grade level over the past five years. The improvements in student performance, however, haven't returned to the high levels of 2007.

The study by Princeton, N.J.,-based Mathematica Policy Research was conducted, in part, to determine how KIPP fared through a $50 million expansion grant that the U.S. Department of Education awarded in 2010. During that stretch, KIPP's enrollment across the U.S. roughly doubled to 68,000 students, including 12,500 in Houston - where the charter chain opened as a small program on a Houston ISD campus in 1994.

"At the end of the day, the growth was justified," KIPP co-founder Mike Feinberg said. "We need to honor commitments to our current Kippsters and balance that with the ethical and moral mandate we have to grow."

On average, KIPP students moved from the 40th percentile to the 50th in math over two years of middle school and from the 37th to the 44th percentile in reading over the same period. A typical student new to a KIPP high school moved from the 48th percentile to the 59th percentile in math, and from 47th to 54th in language arts.

Nuances of the data show that KIPP schools aren't as successful in their first year as they are later, and that KIPP middle school students who go on to attend newer KIPP high schools tend to show slight performance declines.

"As the KIPP high schools are gaining experience, they have larger positive impacts in student achievement," said Christina Clark Tuttle, a researcher with Mathematica.

At KIPP Houston High School on Tuesday, the senior class vice president, Geovani Ramos, said he chose KIPP in part because he thought its teachers would provide more help in navigating the college application process.

"After I stayed here for my freshman and sophomore years, I saw there were so many opportunities and people to help me find out how to get money for college," Ramos said. "Otherwise I probably would be in debt. I think going here will help in getting into a great college, and also getting a great financial plan."

On Tuesday, though, Ramos and other student government representatives were occupied with a more pressing issue.

"I need to ask an important question," Ramos asked his colleagues. "Will you want wings and pizza or just pizza for Friday?"

"Wings would be wonderful," one student said, as several others nodded in agreement. The food was for an upcoming "lock-in," where student government members would spend the night brainstorming their goals and plans for the school year.

The high school's 640 students take classes in a two-story building in southwest Houston, one of the rare campuses where the building was built for KIPP, rather than reconfigured. Multicolored square tiled floors cover the hallways, with black lockers and black mathematical figures and equations lining the walls.

KIPP employs an extended-school day and year and character education to meet its goals. The schools devote tremendous resources to creating strong leaders and hiring exceptional teachers - an area particularly strained during rapid growth, educators said.

It remains to be seen whether KIPP will ever again reach the improvement gains it saw in 2005-2007, when the impact on math scores was more dramatic than current levels.

"It is a definite decline" in the pace of improvement, researcher Philip Gleason said. "The impacts in those early years were huge."

Earlier research from Mathematica showed that, despite common misconceptions, students come to KIPP academically behind their peers.

But the results - even the statistics that show that about 44 percent complete college - are not enough to satisfy Feinberg.

"We're not aiming at the moon. We want to land on Mars," he said. "We want to get to 80 percent. That's what top-(economic) quartile kids are doing."

Feinberg said he'd like to see students at new schools show improved performance faster, but he cautioned that building schools is a long-term effort.

"For us, this has never been a sprint. This is about making and keeping sacred promises to children's families," he said. "Growth is hard, and I'm glad we were able to keep the promises, no matter the year."

Many critics questioned whether KIPP's results could be replicated on this larger scale. They also worry that the charter school chain is hurting traditional public schools by siphoning off extra funding and higher-performing students.

KIPP's staunchest critics have said they won't be convinced until the charter system replicates its success by taking over traditional, impoverished public schools. That would be the only way, New York University professor Diane Ravitch wrote on her blog earlier this year, to ensure the results aren't "enhanced by cherry-picking, skimming, or attrition."