I happen to agree with yekke2 that learning Yiddish is an important step in intergrating into ththe Yeshiva world, particularly in Eretz Yisrael, where many shiurim/shmuezem are given in Yiddish.
But still, I don’t think that justifies giving lessons exclusively in Yiddish to high-school, let alone elementary-school, students who have little to no exposure or knowledge of the language. Why should these students lose out on years worth of material? If a Yeshiva believes that learning Yiddish is important, separate Yiddish classes should be given, so students will not lose out on Chumash/Mishnayos/Gemora while they are still new to the language.