Since this is the premise for the rest of your post, are you saying homeschooling is always better than Catholic school or just that the SSPX is basically equivalent to any other modern Catholic school?
I'm saying that:
1. SSPX schools might be for the most part orthodox (not the schismatic) and kosher (not literally I hope). Seriously, they don't have a lot of outright error, but there are downsides. SSPX schools are not without their problems, being as they are IN THE WORLD and drawing FROM THE WORLD for their teachers. They usually don't have enough money (due to lack of students) so they hire young ladies just waiting to find a husband and get married. By the time they have some knowledge and/or experience, they found their husband and they quit their teaching job.
The problems become much worse when the school wishes to compete with worldly academic institutions... Let's just say they might hire the wrong kind of teachers.2. Many (most?) SSPX schools are a joke compared to the pre-Vatican II parochial schools they are trying to emulate or replace. I'm not old enough to remember them personally, but based on reading and talking to older individuals, academic standards and curricula at SSPX schools are a complete joke compared to what could be had at a Catholic school in the 1950's. This runs closely parallel to SSPX Seminary curricula vs. what used to be normal at a Seminary pre-Vatican II. Ladislaus can elaborate on this one.
3. It's hard to teach groups of kids as effectively as a mother/father can do one-on-one -- when such an option is possible. One-on-one attention is always going to result in the most learning done. That's why private lessons are more expensive than "small group" lessons, which are more expensive than regular classes. Not everyone has the money, time, or ability to homeschool -- but that doesn't mean homeschooling isn't the best.
4. Jesus Christ was homeschooled.
5. The PRIMARY responsibility for raising and educating children rests with the PARENTS. Not just education in the Faith, but in everything. It's one of those inalienable rights that parents have. As in, if the State tries to take this away, it's time to fight. Now parents are free to delegate that task to the Church (parochial/private schools) or even the State (God forbid, since about 2000), but it's still the parents' responsibility to educate their children. No one else's. That's why it's laughable that SSPX priest(s) are claiming mortal sin for those who chose to NOT avail themselves of SSPX schools. IT'S THEIR CHOICE.