Public schools, I assume, vary widely, and my experience with our local public schools may well be different from 4th Watch's experience with the public schools in Idaho. Still, I feel obliged to speak up, since my experience here has been mostly very positive.
I live in a small town (about 20,000) in New Jersey, and our local school system is correspondingly small but not too small, about 1700 students for preK-12. This is big enough to have AP classes and extracurriculars, but small enough that students don't get lost in the crowd. Sure, there have been some mediocre teachers and some personality clashes; but there have also been gifted, skilled teachers who have educated my children better than I ever could. My degrees are in STEM fields, so while I could have homeschooled my children in math, I couldn't have taught them English literature or history or violin the way their high school teachers have done.
The school, like our town, has students from multiple ethnic and religious backgrounds, and it makes a conscious effort to be respectful of that variety of cultures and beliefs. My kids report that they have had interesting, intelligent conversations about religion with their classmates who are Catholic, conservative Protestant, liberal Protestant, atheist, Jewish, Muslim, Hindu, Sikh. I'm glad they've had that opportunity. I'm glad the students learn from each other, and challenge each other to think about what they believe and why.
Addressing some of the specifics in the original post: I'm very happy with the sex education program here, which gets pretty explicit about subjects like contraception by the high school grades. We've talked openly with our kids about sex, but sometimes it's hard to talk to your parents about personal stuff when you're 14 or 15. The teachers belong to a union; the union fights for things like decent pay, good health care, and maternity/parental leave -- I think the teachers ought to get all of those things.
My one reservation about the (American) public schools is that they cannot teach Bible and Christian theology as a school subject -- for a good reason, of course, but I can see why parents would want to choose a private school or homeschool to have more freedom to teach religion as part of the curriculum.
With that said, I disagree with the call to remove all Christian students from the public schools. If your local public school is academically strong, properly funded, and respectful toward the students' religions (including Christianity), a public school can be an excellent choice.