The statistics have shown for decades that peers are what determine political alignment. The answer is therefore simple: don't send your kids to conservative education.
I went to public school in a blue state and it was not far left. The labor movement was taught as a handful of very bad situations that caused workers to strike and peaceful protest fixed it. Things like the Battle of Blair Mountain and the violence it took to get where we are were ignored. Same with civil rights, MLK Jr gave some speeches, some people marched, there was resistance, and then we fixed it.
Any non capitalist leanings were ignored or minimized, the organized violence of the state and those who opposed it was ignored, figures were lauded and their life summaries always left out the part were they criticized capitalism or the complacent middle class. No mention of Mother Jones, Smedly Butler, our involvement in Iran as a pre shah state, or anything that would tarnish America's image as a modern moral state. Hell, they never had the nerve to call what we did to the Native Americans a genocide.
I'd disagree with that. I see that the gains we've made in quality of life are often the result of literally fighting for change. The systems we live under are the result of incremental change over a long time and should be questioned and resisted when our ability to live a full human life is threatened. The systems we built 10 years ago are unlikely to be perfect, nevermind 200 years ago. There's a reason the US constitution has the amendment process. It is a living document we are ment to change to address the problems we face. Knowing what it took to get the changes in the past let us weigh whether the changes are worth it and the ruling class knowing we know our history means they know what's on the table.
If you choose to call living a life with a degree of awareness and the ability to be more than a profit generator far left so be it. However, I'm very fond of not working 7 days a week, 12 hours a day, and dying from preventable illness due to gutted safety standards.
Oh for sure, Private Schools are a plague of dogma, but the most conservative group in France on average would be a centrist in the USA, where the article was published. Compare educated groups with uneducated groups, and it's pretty clear that education leads left.