I understand your frustration. I've been frustrated in the past when I'm hit with something I didn't know, which I feel like I should have known. I feel stupid and/or betrayed for no one having taught me.
But how do I say this -- you not having heard of something does NOT mean it's not Traditional. Unless you are omniscient, that is. See where I'm going with this?
It's very easy to pack on years just living in ONE place, going to ONE chapel, with ONE priest...you get the idea. How many Catholic books have you read? Is it over 1000? How many books on the Church, Church History, or in particular Church Liturgy?
We all, even life-long Trads, have more that we can learn. You stop learning when you die, that's what I always say.
For the record, Triduum is completely Traditional. It just means "three days". It's not the same thing as Holy Week, which goes from Palm Sunday to Easter Sunday. So people needed a word to describe just the "main part" of Holy Week, those Three Days of prayer before Easter. Hence the Latin term, Triduum.
Those who say "Triduum" also say "Holy Week" on other occasions. There's no dark cօռspιʀαcʏ at work. No one has "replaced" Holy Week with Triduum. It depends on what you're talking about: the three days, or the whole week.
The reason your Novus Ordo homeschool group is using it all the sudden? Who knows. Maybe some parts of Tradition are getting quite popular, with the FSSP and SSPX becoming ever more mainstream and accepted by the Conciliar Church and the World. And people are reading much more LifeSiteNews and much less CNN and MSNBC, due to recent persecution/censorship of all things conservative. Maybe LifeSite or other "conservative Catholic" mєdια orgs have been using the term a lot? Just some ideas.