Just because people wouldn't go to school doesn't automatically mean they are uneducated.
It means they have the option to choose what they want to learn themselves.
The curriculum is so standardized that over half the people in school don't care what they learn anymore, and most people forget what they learned last year anyways.
More importantly, the act of teaching us what they think we should know for the larger part of our lives decreases our independence and demotes learning ourselves. By teaching us things, we end up blindly following the curriculum instead of learning the things we really want to learn, and we end up forgetting how to learn things without someone teaching us.
The main argument for going to school as of the present day is that you need to get good grades in order to get a good job.
This is a very flawed argument, because the entire system is inter-dependent. If they completely changed the school system so that we learn what we want our own way, then the job requirements would change from grades to other things, being more focused on skills, abilities, and experience rather then your achievement at tests. If nobody went to school, it would be practically easier to get a job and head to work.
Math I just picked as a random example because I thought it was amusing. If over half the world believes that 1+1=3, and nothing will ever change their beliefs, and they invent an entire new way of approaching the logic of math, then the people who believe 1+1=2 will be wrong in that society.
Grammar hasn't been around forever. Punctuation, grammar, spelling, all of that was disregarded until someone basically invented them. People could still understand each other back then, because not having these things was perfectly normal. Grammar then continued to evolve throughout history and became what it is now, and then the schools and education system locked into place without letting it evolve further, Who really care where the apostrophe goes, whether to use who or whom? The only reason why we expect people to use correct grammar is because everyone else does.
If nobody used correct grammar, then we would have learned a long time ago to comprehend the people that don't, rather than bash them and tell them to use it.
Also, I'm largely drawing hypothetical situations, and I know that our opinions may be different. I accept this, and I am merely offering my point of view. You may disagree with me, but this is what I believe, and I'm not going to change my views.