Seattle doesn't have nearly enough density to have that many high schools. Given the ever falling birth rate, it likely never will. The density would need to be obscenely high.

Figure high schools have 4x the number of students that elementary schools do, and we'd need 2x the elementary schools to make them all walkable, that means Seattle would have to have 8x the number of students in high school than it currently does to justify building enough high schools to make them all walkable.

Not to mention Seattle couldn't really afford to build that many schools, as the city can't even maintain the current schools to basic levels of safety (ongoing lead water pipe issues...)