I recently read a comment on a Hacker News post that really struck a chord with me. To summarize, schools try to cram mastery into a particular allotment of time, instead of letting students work as long as they need to until mastery. In a system without this restriction "high aptitude" children would be able to reach advanced subjects without having to do it externally, and receive the support and structure which is the whole point of formal schooling. Simultaneously "low aptitude" children wouldn't be pressured and rushed. Those who have different learning modalities and would otherwise slip through the cracks would have the time to figure this out before being spit out of the school system feeling bad about themselves and not having found their optimal learning methodology.
This gels with Robin Hanson's notion that the school systems in the most prosperous nations are vestigial artifacts of training for 19th century industrial society. For that purpose the various aspects of our schools seem most appropriate: start earlier than needed, last longer than required, training to do work on command whether useful or not, etc.
School shouldn't be an onus, or a badge of accomplishment. It should be what it's meant to be: a place where maximal aid is provided in the students' pursuit of learning about the world. No need for "We must prepare students for the real world / for work / to be well rounded individuals / for college" sloganeering.
I would like to enable learning by doing one simple thing that shouldn't change how schools are actually run a whole lot, but may feel strange: No more time-based grades, you stay in a class until you pass it*.
* if you want to enforce standards, just select a minimal set of classes that must be finished and a few elective slots, that's how must highschools work already. And if you're scared of classes that have 10 and 17 year olds together you could probably segment classes by rough age ranges.