Salt and Light?

I just finished a really good book by R.C. Sproul, Jr. called When You Rise Up: A Covenantal Approach to Homeschooling. In one interesting passage, he explains why he is unpersuaded by the argument that Christian parents should send their children to public schools to be salt and light:

First, there is always a line drawn. I've never met a parent who determined to send their teenage child off to a brothel or a crack house for the sake of the lost. The people there are as lost as the people at the state school. The only difference is, in the brothel or the crack house, the bad guys don't have the authority to make our children sit and listen to their worldview being taught for seven hours a day. ...

[Secondly,] I have yet to hear of a parent who is so concerned for the lost that they actually pay to send their children to attend a Muslim school, or a Roman Catholic school. Isn't it at least suspicious that all those who are motivated to send their children out as missionaries send them where it's "free" to attend?

Do I care about the lost? Of course I do. Do my children care about the lost? Enough that they pray for them at school, out loud, every day. I am homeschooling precisely so my children will be able to know, recognize, and love the enemy, while not becoming the enemy. And just as their ability to love the enemy into the kingdom isn't contingent on their being trained by the enemy, in like manner their ability to love the enemy into the kingdom isn't contingent on their being in the enemy's schools. The greatest thing our children can do for the lost is to so let their light shine before men that they glorify their Father in heaven. ... I don't want their bushels buried. But neither do I want their flames extinguished. Never will I put my children under the authority of those who are enemies of the gospel, who despise the lordship of Christ such that his name cannot even be mentioned. ...

[T]he most grievous error we can make is to send them off to schools where Jesus is not plainly, fully, and publicly honored. ... May it never be said again of any of those who name the name of Christ that they rendered unto Caesar the things that are God's – his covenant children.