I'm surprised that they aren't homeschooling with that logic. Similarly, I've heard plenty of TBMs talk about how they are teaching their children that the Lord comes before everything and anything, including their parents. In one case it was was staying home on a Sunday instead of going to a water park with their dad.
It's frustrating to see and hear parents teach their children something that you don't agree with. On the other hand, I know my grandparents (some other relatives and probably neighbors) are all frustrated with me for not taking my children to church or even having them blessed with they were babies.
I agree it's a silly reason to skip school. In the grand scheme of life, it won't make a big difference one way or another. It's actually funny if you think about it-- skipping school for an open house. I feel sorry for these kids that are so entrenched in the Mormon church and culture.