THE SCHOOL BELL RINGS and then there is silence.

It is a Wednesday morning in June at the Roxbury Preparatory Charter School and the third day of summer school. The students — clad uniformly in blue shirts, khakis and, for the boys, ties — step out of their classroom doors, line up, and move into their next-period classes. No roughhousing, no shouting or gossiping, no slushies to the face. And these are the recalcitrant ones, the 40 or so kids who punched their ticket to summer school by failing a class during the regular year.

Talking in the hallway, it turns out, is against the rules at Roxbury Prep, be it summer, fall, winter, or spring. The point isn’t so much to churn out monks or suck the tween life force from middle schoolers; it’s to maximize every second. If the students are silent, they’re more likely to be settled when they get to class, allowing teachers to launch immediately into their lessons. “You probably net anywhere between 35 and 45 minutes of instructional time a day,” explains codirector Will Austin. The transition time between classes is five minutes. “But I’m thinking of shortening it to three,” he says, leaning forward. “I think we can do it faster.”

A few minutes later, he walks into an eighth-grade algebra class of eight students. Standing at the back, he points to two boys, Raymond and John. Seated together, they’re working out equations on small dry-erase boards that they hold up for the teacher, Jami Therrien, to check. “I can tell you right now, Raymond failed history, but he’s an excellent math student,” Austin says. “So Jami, being an awesome teacher, has him paired up with John, who didn’t pass [math].” (If a student fails just one class, he or she is required to take the entire summer school curriculum.) By partnering, Raymond and John will help each other. Therrien later points out that John’s next math teacher, while preparing for the new school year, will go back and review last year’s tests to see which areas gave him and other students a hard time, and then work them into the lesson plans.

With all that attention to detail, Roxbury Prep, despite its location in a relatively poor, violent neighborhood, has become one of the highest-performing schools in the state. It runs sixth through eighth grade, and its 265 students — admitted via lottery — are all minorities; last year more than 70 percent of them were disadvantaged enough to qualify for free or reduced lunch. As a charter school, Roxbury Prep is accountable only to the state, not to the Boston public school system, and therefore can employ nonunion teachers. This means a long school day: Classes start at 7:45 a.m. and end at 4:15 p.m., though most kids stay until 5:30 for tutoring, clubs, or (alas) detention. And for three weeks each August, teachers return to the school to prep for the upcoming year.