I am a teacher in the public school system. Are you saying that it is sinful for me to be there?
The Biblical direction for a mature adult Christian who teaches in the public school system is completely different than a child who is learning how to think.
It is possible for an adult Christian to be salt and light in a dark world. However, the Christian teacher may be faced with being charged by the school system to teach children things that are in direct opposition to the teaching of the Bible. Individual public schools are often quite different in what degree they enforce the government teaching of things like same sex parenting, homosexuality, and evolution. Each Christian teacher must evaluate his own situation in light of Biblical principles, and pray before the Lord to discern what is pleasing to God.