The SSPX prioritizes their own good over the good of the Faithful. They LIKE to procure both, but whenever there is a conflict, the good of the Organization comes first.
So the calculus is simple:
Homeschool: benefits souls, but not the SSPX at all
SSPX School: benefits souls*, AND the SSPX (by having your kids minds, a huge opportunity to mold their thinking)
Their choice is simple -- at least for them.
*Of course, that's assuming their school DOES keep your children away from the worst The World has to offer. But having read about the scandal in Post Falls, ID (a good number of children sodomizing each other) I have to say that "SSPX school" doesn't ALWAYS tick this checkbox. For more on the Post Falls scandal, there is a huge thread on CI about it:
https://www.cathinfo.com/sspx-resistance-news/sodomy-scandal-in-post-falls-id-sspx-immaculate-conception-academy