When i say MO schools, I am referring to schools that have a
Greater emphasis/more time on secular studies than kodesh, a mixture of boys and girls, a cultivation of a non/anti torah approach to ethics, civics, politics, and ideology, and other hallmarks of modern orthodoxy. Zionism is a separate issue, but fits into the last qualifier for the most part.
There is a place for kiruv yeshivos, or as you described, private schools with a sprinkling of judaism for parents who otherwise would put their kids in fancy prep schools. That doesn’t mean that their education is valid or a serious attempt at transmitting Torah to the next generation. Good kiruv yeshivos know how to make the most out of what little time and attention for kodesh that the kids will put in.
There are many yeshivos which cater to baalabatishe jews who want their kids to be ready for college, but this isn’t their main focus. Their high school years are filled with torah as the first priority.