In what circumstances, if any, do we teach our kids that it is good to fight back?
There is a big difference between defending and protecting oneself from severe harm and seeking revenge. Differentiating between these two extremes is very difficult for a child. That is why Christians have maintained a standard of not fighting back. Children, by human nature, will almost always lash out at their enemy, even if they are not in any danger. They become angry and unreasonable in the face of a wrongdoing directed toward them.
In Matthew 5:39, Jesus says, "But I tell you, do not resist an evil person. If someone strikes you on the right cheek, turn to him the other side also." This principle that Jesus was teaching is that we are to do good to those who come against us, not seek revenge. Because it is so difficult to teach a child the type of self-control required to make a quick determination of their true motives, we usually just teach them not to fight back at all. Better than that would be to teach them how to defend themselves if necessary, but also how to glorify God, not themselves, when doing it.
Biblical principle aside, in this age of violence, a school record can be completely ruined by a suspension or expulsion because of violence. And these days, it doesn't take much to warrant those types of punishments. That reason alone is more than enough for me to teach my children that there is no place for violence. So, what must be taught to kids is that they can defend themselves without allowing anger and violence to control them.