That's not even remotely true. Most major metropolitan school districts in my state (California) cater mostly to the mediocre, and offer advanced classes to some kids who are better at following instructions, and minimal training to problem kids. The control ought to be more fine-grained than this. The problem kids, for example, are all just lumped together, even if no rational person would think that prudent. For example, at one school I know of, the mentally disabled kids are placed with kids who are just too far behind other kids of the same age, and both groups are blended with kids who are just disciplinary issues, with no ability on the part of the teacher to deal with each situation in an appropriate way. Thanks, standardized testing!

Of course, the modern purpose is something quite different now. For example, in California, we get Penal Code Section 1170(a) which says, in part:

The Legislature finds and declares that the purpose of imprisonment for crime is punishment. This purpose is best served by terms proportionate to the seriousness of the offense with provision for uniformity in the sentences of offenders committing the same offense under similar circumstances.

Whether this sort of thing has any tendency whatsoever to reduce crime is subject to debate.