At my sspx church you have two types of families, ones who send their children to the school and ones who don't.
There are around 4 families who don't at the moment, almost all of them have true mary like modesty, the husbands wear suits at mass (except one), and their children are better behaved and seem smarter.
Then there are dozens of families who sends or sent their children to the school and almost zero are modest at all, I have heard their children talk about video games and movies etc. And most of the older parents whose children are out of school only have one or two children at mass... now lets say for example one had 10 kids who are grown, 2 are at the church, one is a priest, where are the other 7? Did they all move to other cities? One family I know well had 4 kids at the school, 1 is no longer catholic, one dresses like a hippy and doesn't go to mass regularly, and one thinks he is gαy and doesn't go to mass. So from what I have seen I could totally see 40-80% of children educated at sspx schools leaving the faith.
The factors I think that cause this is the crises in the church is not taught well at all, the novus ordo church is not taught as evil, but as a little brother in need of help, you have mostly women in their early 20s teaching who are not well educated or smart while the pastor only teaches religion class, the evils of the outside world are not stressed nearly as much as they need to be, the parents send them to the school because they themselves are not super strident catholics 7 days a week and they don't teach there children anything extra because they think the school is wonderful. And at the school it is stressed that they should go to college! So all the bad factors above get flayed out when the go to college and then they become a novus ordo or a agnostic catholic, or confused and depressed.