Yet my experience has led me to believe that K-12 didn't matter.
I learned to read at home, so they didn't give me that. I was constantly stifled; from my perspective as a child, I saw that my own interests were almost systematically kept out of reach (as an adult, I can see far further as to why and how this situation came to be - but at the time, all I could do is to see their chains and rebel). I was routinely in trouble from sixth grade on up. Remember those end-of-year class trips you went on? They never let me go because I was such trouble! I graduated high school with a D average.
The one thing they did do is to set back my educational development by about three or four years. Once I was finally free, I had time to grow unhampered. I discovered that learning was actually the opposite of prison, and that it had everything to do with the things I was interested in. I studied all kinds of things independently, and when I was ready, I took a couple of calculus courses at a community college to prove that I was serious. On that basis, I was able to gain acceptance at a well-respected university, where I majored in molecular biology and graduated with distinction.
If proponents of public education really cared about what was best for me, they would have left me alone. My experience shows that a K-12 education wasn't necessary to prepare me for college, or for life.
Edit: I'm not saying that no child benefits from their K-12 education - it is clear that some do. But why must it be involuntary? What justifies the laws that compelled my parents to send me to a place they knew (at some level) was hurting me?