Salt and Light in the Public Schools

You are sitting around the dining room table when the family you're eating with announces: "We are sending our 9 year old son into the jungles of New Guinea to evalangalize the pagan headhunters!" You don't know what to say - it sounds so pietistic and holy.

Somehow, their evangelism scheme doesn't seem like it will work. Yet, sending children into public schools to be missionaries is probably the #1 rationale used by Christians who support public schools. Is this really an effective way to reach the lost, and at what cost?