That depends. so I had to go with maybe.
If there is a serous inequality between the fighters, then yes, someone should break it up. However if it is not a case of someone stronger bulling the weak, then it should be allowed to continue.
Unfortunately, the local schools in my area of Texas "punish" everyone involved in a fight. So the crowd definitely won't step in and the person defending him/her self gets "punished" also. This is obviously biased treatment inspired by people who believe that all violence is wrong. But, I believe it to be wrong. Children should always be taught and encouraged to defend themselves and those less capable of defending themselves.