Equating underperforming students with disciplinary problem students seems to be a common problem in this thread. There are many underperforming students who would perform much better without being subjected to a threatening or harassing environment. Public schools attempt to provide universal education, which is at direct odds with bad-faith students that poison the well. In fact, in some places where this is legal, public schools get better when they can expel criminal minors into the charter system. Separating the worst offenders from the other students might also match these students with resources that are most equipped to help them (eg. counselors that might help to reduce gang violence).