1. School curriculum structured more like an RPG skill tree. The main point or advantage of this is that students can take any path they'd like to be "Level 12th Grade." Many schools let people into physics who don't know algebra. This, for example, would result not only in kids not learning physics, but also becoming less enamored with learning themselves due to the inevitable total failure they'll experience. .
Obviously, as a by-product of this we'd have to get rid of grades. Rather than grades you simply can take courses, and once the "cluster" of requirements are completed you'd be deemed to be "nth grade."
2. School should ideally be intertwined with practicality. For at least K-12 I see little reason why everything can't have a more practical focus. This removal in abstraction will keep students engaged and make more clearly the point that everything they're learning is actually useful. With this being said, schools should actually emphasize teachers less. Kindergarteners/1st years and 12th grades/final years should be the only ones to be taught by teachers directly.
Ideally, everyone else would be taught by the year above them. Teachers would be present, but mainly to oversee curriculum development, and keep things on a higher level moving along. This has the nice advantage of making it so that in 2nd - 11th grade, student "teacher" (who's just another student) is 1:1. With this kind of attention most students on the cusp of learning will finally be able to get the individualized attention they need.
The main problem with this is if the students aren't smart or knowledgeable enough themselves to teach anyone else. This goes back to (1). Students should be ideally, excellent or out. The idea of passing grades is kind of silly. Passing should be excellence. I wonder why people think it's OK to just get by in school? Everyone is capable. School isn't [that] hard.