Which they can do outside of school just as easily.
Just as easily? No. You evangelize people you know. When the world sees your joy they will want to know the secret. The secret is that "Christ is the joy of our salvation." Evangelizing strangers is very difficult especially at that age.
At the college level: I was Evangelism Chairman at Jacksonville State University Baptist Student Union which was strange we even had the position because back then, in the late 70's, evangelism traced back to the Baptist Student Union would threaten our existence on campus and we might even have lost us the building. Couldn't pass out tracts or anything.
Today, there are many denominations present on or very near campus and are allowed to advertise on the campus electronic bulletin board right on the front lawn. Times have changed for the better here, in that regard.
So I know at least a little bit about evangelism. I have served on my churches evangelism team, going out on thursday nights to visit homes where cold hearted family members would allow us to speak to them and pray for them for a few minutes and rarely had any success. Once people cross a certain threshold in life it becomes very very difficult to reach them. "I am almost persuaded."
This is one reason why maintaining a presence in the public schools system is best for the LOST kids. People in this forum have argued that it is best for Christian kids to leave, and they could have certain points about that, but I can't allow those points because the lost kids will have such a limited chance to receive the light of Christ when the Christians are gone. Their main influences in life will be the pot-heads and the dysfunctinals who are headed straight down the path to destruction. I know what that result will be, it will be lost generations. If you begin to feel outnumbered today, consider how hard it is going to be a few generations from now if we hide ourselves in private schools.
Yes, the public school systems need our prayers, our thoughts and we need to keep up the efforts that are already in place to reach more souls for Christ (ie: summer Bible schools (VBS), after school programs, invitations to church and sunday school.)
Not allowing the kids to play Silent Night in a school play, however frustrating that may be, cannot be allowed to stop us from doing the work of Christ.
"Put on the whole armor of God that you may STAND."