Here is a question that I've seen asked in debates over public education. One side says parents should leave the public schools because they are a bad influence on the kids. Then the other side asks, "Well is it a sin to send kids to public school?"
The point in asking this question is to put the anti-public schoolers in a bad position. If they say that sending kids to public school is a sin, they will be charged with being "extremists" and going beyond what Scripture says. But if they say sending kids to public school is not a sin, then why are they holding such a strong position?
However, there is a problem with this question, which is that not every question in life involves the violation of a direct command of God. Many issues in life are questions of wisdom. For example, take the question of whether a person should take a job offer to do _______. There are some circumstances where the option is flat-out sin (e.g. performing abortions). But what about working as a divorce attorney? It may be difficult to call such work "sinful" for a Christian. However, one can make a strong biblical case that it is a bad idea for a Christian to regularly engage in such work.
The same goes for sending kids to public school. Is it a sin for Christians to send their children to public school? Maybe. There are many different circumstances out there. Consider a wealthy Christian family who has a great Christian school in town, but they choose to send their kids to public school just so they have more money to buy a new boat. That situation is much worse than a working single-mom who has no Christian school in town. She's in a tough spot, and everyone understands that.
The question of where to school children is a wisdom issue, and there are several factors at play. However, this does not mean there is no opportunity here to make really bad decisions. And some of them may in fact be sinful decisions.
It certainly is foolish to refuse to give an ear to those who raise concerns over the godless environment of modern government schools. The vast majority of American public schools are pushing an anti-Christian and immoral worldview on students, and the academics are generally lousy. The end product of today's public schools is simply frightening.
Thus we come back to the question often raised against those who argue that we should not send kids to public school—"Well is it a sin to send kids to public school?" This is analogous to a man telling his neighbor that he should not let his children play in the busy street because several kids have been hit by cars there. Then the neighbor responds, "Well is it a sin?"