School is about teaching your brain how to retain information in general, so you can retain what you need to use.
They would lack any info about world wars, decolonization, computers, space race, internet, climate change, just to pick some topics taught today.
The people who built all those things and won two world wars were educated with this curriculum. They took us from horse and buggy to space travel, clearly they were doing something right with education.
More broadly, home schooling seems to be more effective in 19th century America. Edison likely never took any test and definitely never attended a college that depended on it. He was home-schooled because his hearing was so bad.
It's true though that if you're learning math the focus is on learning math, not theorizing new branches of mathematics. The reason being that most topics are cumulative. You can't study abstract algebra if you didn't pay attention in high school algebra. In that way, they force conformance. But largely I think this is a good thing.
This sort of thing also applies to English/Language Arts. In order to comprehend more complex media, you have to be able to comprehend simple media. Often, I hear people lament about how school didn't teach them about real life. Typically, the reason why is because they didn't pay attention in English class. There's a lot there.
Also, intelligence is virtually worthless without knowledge. Intelligence just describes potential. If you don't use that potential, it's no different than if you had no intelligence at all.
School mainly serves as day care and social programming: obey authority, believe what we tell you, remember what will appear on the test. Some people get more out of it than others.
If you've got a system that can reliably take a citizen earning $40,000/year and turn them into a citizen earning $140,000/year, governments from all over the world will beat down a path to your door. Over a 40 year career that's an extra $4 million in taxable income.
I can think of many other examples of the government fleecing citizens into poverty (medical care, just to give one), enriching a few corporations and wealthy individuals at the expense of the general welfare. A government that cared about its citizens making more money wouldn't have hundreds of thousands of them living on the streets. If the government actually prioritized increasing tax revenues I can point them to some very rich companies and individuals not paying their share.