They only "dropped" it in the English name, probably because of the length and maybe because it might be confusing for people with 0 knowledge of the Japanese language. They're still 新しい学校のリーダーズ in Japan.
While it seems appropriate given their anarchic approach, it doesn’t mean New School Without Leaders, it means Leaders of the New School. As @BreathlessGust said, that could be confusing for English speakers. It might also be problematic, because that happens to be a Japanization of the name of an American rap group from the late ‘80s - which was probably intentional at the time. The original Leaders of the New School, which featured Busta Rhymes, were pretty funny damn funny, too.
