At College Preparatory Academy, it started with a demerit. If a student talked back, teased another student or wasn’t following the public charter school’s policy on uniforms, he or she would get a demerit, worth 30 minutes of detention to be served after school. More demerits, more time.

But if the student blew off detention, he was suspended. Usually for just one day, said Susan Schaeffler, CEO of KIPP DC, which runs College Prep and nine other schools in the District.