1. I believe many schools gave up teaching in Yiddish since they ended up teaching Yiddish rather than teaching Torah. Where you have Yiddish speaking communities (i.e. where the parents speak Yiddish to the children, and the children speak Yiddish when speaking with the friends), the parents will probably object to non-Yiddish speaking students in their schools. We probably need frum materials for teaching Yiddish as a second language, since most existing ones are teaching the “secular” Yiddish that existed 80 years ago, rather than the modern Yiddish spoken largely by Hasidim in the 21st century.
2. The common language of Jews has always been Hebrew. Jewish secularists were desperate to find an alternative to Lashon Kadesh, but even in the pre-WWII period, Hebrew remained the common language and the language of Jewish scholarship and learning.
3. All living languages have dialects. Yiddish was considered a German of dialect before World War II. Arguably the zionist “Ivrit” is no more than dialect of Lashon Kodesh, and in Eretz Yisrael several dialects clearly exist (with differences in vocabulary, grammer, word order, and how certain letters are pronounced).