Arizona-based BASIS charter schools consistently rank among the top public schools in the country, but U.S. Department of Education data show the schools struggle to retain students through high school.

Critics argue the small graduating classes give BASIS the appearance of a school system that succeeds in creating top scholars out of nearly all of its students. They say the lower-performing students transfer out of the system before senior year.

"There is a small subset of students who can achieve at BASIS, and they need to be highly motivated and academically gifted," explained Julie Erfle, whose son transferred out of BASIS to attend high school in a more traditional setting. "And without that, they are going to have a very difficult time."

BASIS officials reject that notion.

"We’re not expecting our students to be uber geniuses when they walk into this building," Tim Eyerman, the head of BASIS Phoenix Central, said. "We’re not even expecting them to be uber geniuses when they leave this building. Grit and determination measure far more in life than being a genius. There’s plenty of geniuses that fizzle out pretty fast."

BASIS arranged for interviews with a group of parents whose children attend BASIS. They all said they were very satisfied with the program, and that they did not believe the school was too challenging.

"My children are regular children," said Bridget Querciagrossa, who has three children at BASIS Phoenix. "I don’t find that the amount of homework is overwhelming."

"Our conversations at the dinner table have changed," said Melissa Penniman, whose daughter is an eighth-grader at BASIS. "I mean she has interesting things to say every evening."

"My senior is getting all sorts of colleges writing to him and begging for him to apply," said Amy Anderson, who has two children at BASIS.

But statistics from the U.S. Department of Education show a large drop-off in student enrollment from middle school to high school.

During the 2012 to 2013 school year, BASIS Scottsdale had 144 students in the sixth grade, but only 32 in the 12th grade. Other BASIS schools show a similar pattern.

"Those nine to 20 to 50 12th graders at some of our newer schools didn’t start with classes of 100 kids. They started with classes of 20 kids or 30 kids," said Peter Bezanson, the CEO of BASIS schools.

He said the schools are gaining enrollment in the lower grades, which should translate into larger graduating classes. Bezanson argues that actual drop-off is roughly 10 percent per grade, except between the eighth and ninth grades, where the attrition rate is 25 percent.

"We don’t like that. We’re working to, we want to keep all those kids,” Bezanson said.

But critics say there is a problem with the way political leaders and policymakers view BASIS and compare it to other schools.

"You can’t compare it to the other schools around,” said Amanda Potterton, a PhD student at Arizona State University. She has studied BASIS and the makeup of its student body. She said BASIS has few students who live in poverty and few students with special needs.

"The data show the ones who stay are going to perform well on tests no matter where they go to school," Potterton said.

She and others argue that the school system’s ranking is inflated because it teaches only the brightest, most motivated students, while traditional schools teach students from a variety of backgrounds and with a variety of problems.

"I don’t think we should be touting a school that graduates a couple of handfuls of kids as somehow superior to these schools that graduate hundreds of kids with many different challenges," Erfle said.

But there is a large demand for seats at BASIS desks. There is currently a 7,000-student waiting list for this fall.

Copyright 2015 CBS 5 (KPHO Broadcasting Corporation). All rights reserved.

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