Shouldn’t this be the present perfect “has followed the directive until now” because the directive was issued in the past and has been followed until now?
That's a good point, but there is a reason why the past perfect and not the present perfect is better in this case, and that is because the situation has now (already) changed: the Osaka prefectural (municipal) government used to follow the (national) education ministry's directive banning mobiles and smartphones at schools, but it no longer does.
The present perfect is used to refer to an action that started in the past and has effects on or continues up to the present. The "now" in the article is not synonymous with "this very moment" (in which case the present perfect could have been used), but extends a little further back, possibly to the moment when the local government changed the law on the use of mobiles at schools (at some point in the immediate past), which is why the past perfect is correct. "until now" would thus mean "until a few days/hour ago (when the regulation was locally changed."