Preamble
I’m going to talk about the different type of k-12 schools, and colleges there are. Mind you I attended a private university, public college, public k12 school system for a while, and private k12. So I might have some biases.
k12
Public
So, public k12 schools quality seem to vary by the location. From my personal experience my k12 education wasn’t great. There are non profits that pay teachers more money to decrease the turnover rate(https://en.wikipedia.org/wiki/Math_for_America), and teachers for america that place competent teachers in schools that need them the most. https://en.wikipedia.org/wiki/Teach_For_America
Private
So, there’s different types of private schools. There’s private homeschools, and college preparatory schools(https://en.wikipedia.org/wiki/College-preparatory_school). I personally think that homeschooling the best option in this day and age. It’s cheap (depends on if one parent can afford to be home, or if a tutor could be afforded.), and generally provides a higher quality of education. Many of the IMO, and Mathcount winners were homeschooled. Reid Barton was homeschooled, and later went on to earn his doctorate in Mathematics. There are tons of high quality open educational resources like OpenStax, or Khan Academy that can be used as a homeschooling curriculum. There’s also good books that can be purchased for homeschooling (Art of Problem Solving books etc.). John Holt was an advocator for homeschooling. He attended Exeter (a prestigious college prep school), studied at Yale, taught k12, and servered in WW2. I recommend reading one of his books, or watching one the YouTube videos with him in it. Stanford has a online high school (https://en.wikipedia.org/wiki/Stanford_University_Online_High_School), and some universites have privacy lab schools. Khan Academy just opened their own lab school (https://en.wikipedia.org/wiki/Khan_Lab_School). There are selective private kindergartens https://www.imdb.com/title/tt1270833/.
For profit
Diplomia Mills etc. Avoid.
College
Public
Community Colleges are a great option for a lot of people. You can take general education courses there for cheaper than at a four year college, and then transfer to save money. Community Colleges are funded by the community, or state. Most provide free GED preparation, and a course to learn English.
Private
Some can be worth it. Just make sure that you pick a in demand major. There are colleges that offer cheap or free tuition. Olin College of Engineering, Deep Springs College, Caltech, Harvard, and Stanford.
For-Profit
Diplomia Mills etc. Avoid.