Where in Japan is there a school that makes kids do army crawls in gym class?
Literal
Japan [possessive-の] where [at-に] phys-ed [with-で] crawling-advance do-[causative-させる] school [subject-が] exists [rhetorical-んだよ].
ほふく前進 (匍匐前進, 'crawling advance, army crawl') is a military term. 体育で ('in P.E. class') sets the context. やらせる is the causative of やる. 日本のどこに…あるんだよ is a rhetorical question expressing disbelief — 'where in Japan would you even find such a school?!' んだよ adds exasperated emphasis. The absurdity of military drills in school P.E. is the joke.