There is a difference between self-defense and retaliation. This isn't a situation where the girl is being mugged and has to do whatever is necessary to protect herself. It is a situation where she got zinged, fell down and the situation is over. Retaliation implies either she gets back up and bloodies his nose/blows out his knee/kicks him in the groin etc or the situation is ongoing and she does the same. Neither is correct. Two wrongs NEVER make a right. Once again, to be crystal clear, in a SD situation all bets are off and one needs to be able and willing to use the level of force needed to survive the situation. SD situations occur outside the school in an uncontrolled, unsupervised setting with no rule set being observed. This was not the case in the OP situation described. She was unable to protect herself in a controlled and supposedly supervised setting. She got hurt. She fell down. Situation is regrettable but it is now over. Retaliation = revenge. Civilized people don't seek revenge. Adults should not be teaching children to seek revenge.
Now take a situation where the boy puts her in a headlock and is currently choking her out...it is an ongoing situation. She can then do what is needed to defend herself in this ongoing situation. That isn't retaliation, that is self-defense. But again, in the OP, the situation was over and done. Suggesting that retaliation is a solution, particularly from adults to children is wrong. I honestly can't believe it was suggested or that it is a point of discussion.