I have been following the progress of KIPP public charter schools since 2001. Initially this charter network was just one story out of many. But when its first school here, the KIPP DC: KEY Academy, began performing better than Northwest Washington schools with many middle class children, I made it a regular stop.

I also spent time with the network’s founders, Dave Levin and Mike Feinberg, visited about 40 of their other schools and wrote a book about KIPP, “Work Hard. Be Nice,” published in 2009.

This summer there are 125 KIPP schools with a total of 39,000 students in 20 states and the District. Eighty-seven percent of KIPP students are from families poor enough to qualify for federal lunch subsidies. Fifty-nine percent are black and 36 percent Hispanic.

No charter network has been researched as much as KIPP. (Its initial name, the Knowledge Is Power Program, has been streamlined.) Research suggests that no other collection of public schools, charter or regular, have raised the achievement of low-income students as high or as consistently as KIPP. But that still isn’t getting enough of KIPP kids to and through college. Levin is experimenting with a character growth card that asks teachers to assess, and find ways to teach, personal characteristics like grit, self-control and optimism. Feinberg is building a network of colleges promising to accept a dozen or so former KIPP students each year.

Keeping track of individual schools is the best way to understand KIPP. It offers longer school days and years, better trained principals, teachers encouraged to be creative and unique incentives like week-long field trips. But there is no magic formula that works for every child. Some years are worse than others.

An update on KIPP schools in the District, based on the latest D.C. Comprehensive Assessment System tests, illustrates this. The principal of the first KIPP school, Susan Schaeffler, now directs 10 schools in the city. Every year the test results reveal new challenges.

Last spring, for instance, KIPP DC’s 3-year-old high school, College Preparatory, saw its math proficiency rate plunge from 92 percent to 76 percent and its reading proficiency drop from 78 percent to 52 percent.

KIPP’s three middle schools still score well above the D.C. secondary school average of 46 percent proficient in math and 42 percent proficient in reading, but KEY’s math proficiency has dropped from 91 percent in 2008 to 78 percent this year, and its reading proficiency from 73 percent to 67 percent in that same period. The WILL Academy is mostly unchanged in that period, from 65 percent to 67 percent in math and from 56 percent to 55 percent in reading.

Only the AIM Academy has shown significant gain in that period, from 65 percent to 85 percent in math and from 49 percent to 59 percent in reading. The three schools’ eighth grade proficiency rates remain high, from 85 to 100 percent in math and 67 to 87 percent in reading.

Schaeffler says much of the struggle to keep achievement robust comes from the influx of special education students with disabilities. Critics say charters discourage such students from enrolling, but that doesn’t apply to KIPP. In this new school year 21 percent of WILL students qualify for special education. AIM’s share is 19 percent and KEY’s is 14 percent, compared to the city average of 18 percent.

Schaeffler has the flexibility and teacher talent to try new approaches for such children, but she says the entire system, charter and regular schools, should find a way to cooperate in special education. “We need to change the culture of not wanting special needs students to embracing them,” she said.

That effort will test the strength of any public school, even a charter network that so far is the best we have.