gdoug10, your premiss is somewhat flawed. The school is not responsible for any and every thing that a pupil does, whether on a school trip or in school hours. Its duty is only to provide a reasonable standard of care such as a parent would do.If one boy assaults or murders another, the victim's parents could only sue the school if it had manifestly and negiligently failed to prevent the incident; if it knew, or must have known, that an assault would take place and did nothing by way of preventing it when it was able to do so. (It would be a highly unusual case if that were so; the circumstances would be so extreme in nature that it is almost inconceivable).
Now in what way is the school liable for two pupils fighting in the street ? The school has said, in effect, "Make your own way from one building to another" What do you expect the school to do? Have the two pupils under some tight escort to guard the two lest they take to assaulting one another (or commit arson, or burgle a shop) on the way there? It really wouldn't make a lot of difference if it was in the school, but it wasn't.
It follows that the school can't be liable for the incidental damage (itself only an arguably forseeable consequence anyway) resulting from the combat and you have no case.