IIRC that was mostly the adults, or older teenagers, who don't get a lot of screen time so their designs are simplified?

It kind of follows a general trope in manga/anime where characters' eyes are smaller or thinner to indicate age/seriousness/maturity/intellectualism. The Korean kids tended to have the same kinds of eyes as the Japanese kids, like Hikaru's main Korean rival (forget his name) having almost exactly the same trapezoidal design as Akira. Hikaru's eyes also change into this later in the series, changing from his original carefree wide-eyed design.