Don't try to send them to a 'good' school, certainly not an 'academically good' school. Bluntly, the K-12 teachers don't know anything worth much serious study or effort.
The best can hope for from a K-12 school is that it will do no harm.
Find a school that has a nice 'social atmosphere' and otherwise is RELAXED, e.g., with as little homework as possible.
Then for the real learning, have your children do that at home -- evenings, weekends, summers, and/or school study hall.
What materials? Sure, academically by far the crown jewel is just math. So, get them through math to the college sophomore level ASAP. The next best subject is physics. So, get them through freshman physics based on calculus ASAP. How? Get the best texts (good used copies are FINE, especially for the math) and work through them. If need some help, ask some friends who know the material well and/or find an appropriate, helpful college prof. Or have one parent enroll in, say, a community college course and there learn enough to guide their children and be able to ask the prof questions.
After math and physics, emphasize writing, say, letters, notes, blogs, whatever.
Sure, get 'computer literacy' and also learn the basics of programming.
Then emphasize geography, history, and art history. For English literature, at least learn about the basics of 'drama' and 'formula fiction' and touch on the more famous works. For as many of the works as can, start by watching a movie of the work. For more in drama, understand how movie and TV dramas work.
Somewhere get some basic coverage of 'people and personality', especially about human emotions. E.g., lacking anything else, read E. Fromm, 'The Art of Loving'.
For more, have the child take some community college courses, e.g., biochemistry and biology.
For more, look at the International Baccalaureate program and pick and choose from that or, maybe, just go through the whole thing.
During the last 2-3 years of the 'high school' time, review what else, if anything, is required to do well on the SAT tests.
For more? Maybe learn piano?
Likely could study accounting and take the CPA exam. A CPA would be nice to have.
For more? Jump into college material at the junior level and keep going. Get through college ASAP and then go for graduate or professional school.
For K-12? Don't ask for much and mostly just circumvent it.