Many Christian parents share this sentiment. And it's a noble one, too. Every one of us should strive to be salt and light wherever we find ourselves. But we have to be mindful that young children (before high school) are generally not equipped to defend their faith and influence others. Even with high schoolers, it is the unusually well-grounded student who can impact others for the Kingdom. However, even with the strongest testimony, we would not recommend that "salt and light" be the primary reason for sending your child to a public school.
"How much salt and light can untrained, silent Christians be? Must we not rather train our children in Christian schools that they may become salty and brilliant lights to transform a society wandering in darkness?" - Dr. Wayne Grodin
No matter how well-grounded your children are, it's highly unlikely they can be the kind of salt and light Jesus was talking about. Can fifth- graders, or even junior high students, effectively explain the Gospel? And what about high school upperclassmen? Are they, on average, capable of defending their faith to the degree to which they can be genuine salt and light to their non-Christian peers?
In the vast majority of cases the answer is no. Simply put, most kids are not mature enough for the job, emotionally or spiritually. They are still at a time in life when they need to receive salt and light, not provide it.
And in those rare cases in which a young person might be up to the task, placing your child in public school for that reason would put him or her under tremendous pressure. Sending a child into an environment that is anti-Christian in philosophy subjects the mind of your child to daily teaching that is determined to replace God in every area of life. It's a dangerous experiment, one that could have lasting repercussions.