As the high-profile Knowledge Is Power Program network of schools continues to expand, KIPP leaders are taking a close look at student attrition amid arguments from critics that the loss of students at some of those public schools of choice is alarmingly high.

Attrition rates at a few KIPP schools in the San Francisco Bay Area, in particular, have recently drawn scrutiny. Fewer than half the 5th graders who entered three new middle schools in fall 2003 are still enrolled this academic yea...