Children today are exposed to various kinds of exploitation, both mental and physical. They should be adequately coached in self-defense. Even when they are primary and high school, they are subjected to bullying, teasing, and humiliation. Unless they are prepared to be physically and mentally strong, they will succumb to pressure. It is wise to send children to martial arts classes when will help them protect themselves in tight corners.
However, you should not allow your child to practice violence at school. When classmates hit your child, your child can raise his voice and give a stern message, not to mess with him. He should not hesitate to escalate the issue to his teacher. But if you teach him to hit back, you are allowing him to grow up a law breaker. When there are a means to report the assault and get justice, your child should not hit back. The coaching in self-defense should not help him to instigate violence at school. The techniques are justified when practiced against a set of ruffians who try to abduct him. In such case, your child can very well hit back, since there is no other option to escape. You should teach your child the ability to exercise presence of mind. Hitting back at a classmate is a bad decision, founded on faulty grounds.
There is a very thin divide between being meek and being outrageous. Both the extremes are bad. But hitting back at school is a display of bad culture and upbringing. You are not encouraging cowardice by asking your child not to hit back. You are teaching him tolerance and lawful means to establish his rights. Children must be asked sensible questions when they complain of classmates hitting them. Parents must ensure that the teacher follows up the matter and punishes the trouble makers.