The piece you're missing is the high level of support. You're right that raising standards without also raising the level of support dramatically is a recipe for failure, but that's no reason to write off the kids who don't currently have the knowledge and skills they need. If they need remediation to make up for past gaps in their education, they should get it, while still being expected to learn new concepts as far as they can. As the lower grades improve their net so fewer kids fall through it, the remediation at the higher grades becomes less of an issue.