The classroom of a Christian teacher is to be a place of ministry, not a door to ministry. God hasn't given us jobs to support ministry, but jobs to do ministry. Our work is not intended to be a distraction from the "important stuff" that we do in our leisure time. The time we spend with the Body of Christ in the place where the His church meets in our community is not the focus of His plan. That time is intended to prepare us for the work that we do with all the other people in all the other places in the "normal" world. Sometimes it seems that we spend all our time studying the statistics of our Sunday practices while ignoring the results of the week-long games.
Teaching, learning, and administrating are all workplaces ministries that can and should develop in classrooms and schools as well as homes, Sunday Schools, and churches. Studying any aspect of all that Christ made (Colossians 1:16) is an opportunity to see something of God in what He has made (Romans 1:20). Ignoring the truth He's clearly communicating isn't an expression of tolerance but a distorting of reality that ends in bondage and deception instead of freedom. Christian teachers have a unique opportunity and huge responsibility as their life and words shape the thinking of each new generation whether in a national or private, secularized or Christianized school.
I have been strongly challenged and influenced by the thinking of Mark Greene from the London Institute of Contemorary Christianity. I'd recommend considering what he has to say about A Vision for Workplace Ministry.