Some schools have tweaked those policies after seeing the effects on students, particularly as they exit their charter schools for more lenient environments. Others aim to distance themselves from the harsh practices that have grabbed headlines and generated fears that they could erode crucial political and parental support for charter schools. And some have changed simply because the charter sector’s swift growth has made faithful implementation of original practices impossible.

Still, “no-excuses” ideas still form the backbone of the culture at many charter schools that belong to networks like Achievement First, KIPP, and Ascend, which generally have firm guidelines for everything—from how students walk down hallways to what they wear. Teachers say they’re key to allowing students to focus in class and net high scores on state tests. But as the sector grows—and issues of school discipline make national headlines—many schools are pulling back slightly as they search for the right balance.

“There is a broad movement away from no-excuses discipline policies,” said Mary Wells, the co-founder of Bellwether Education Partners, a nonprofit that advises charter schools. “Most are having conversations about how and how much should we adjust our culture.”

* * *

At the start of a ninth-grade physics class at Achievement First Brooklyn High School last month, students sat in silence as they worked on a problem. They had less than a minute to scribble an answer on their whiteboards.

“Twenty-four seconds,” Alexis Riley warned the class. When her timer went off, students held up their answers and the teacher scanned the room. “85 percent mastery,” she said.

The leaders at Achievement First, a network of schools in Brooklyn, see a rigorous, rule-based school culture as key to allowing students to be as focused as they were at the beginning of physics that day. They also see it as part of a strategy for ensuring that students in poverty do not fall behind more affluent peers.

After all, if students are not required to pay attention, how will they learn the concepts they need to succeed in college-level science classes? If they are not ready to learn within 60 seconds of entering class, when will they catch up?

LA Block, a first-grade teacher at Achievement First Endeavor Elementary School, noted that students sit in silence at lunch for the first six weeks of school. In the hallways, students are asked to walk with attention to the acronym “HALLS”: hands at your sides, lips locked, safely walking.

The idea is that strict guidelines for how students should behave in and out of class, enforced consistently over time, provide the basis for academic progress and can help close the achievement gap. Without them, schools become chaotic—the kind of environment common in district schools that the founders of Achievement First and other charter-school networks set out to create alternatives to.