In the case of the flag belonging to the college yes, it could be destruction of private property. But, the catch is that most of these guys buy and burn their own in order to make their "statement". To me, it should be at the very least incitement to riot, disturbing the peace, blocking a walkway or throughway when there is no permit to demonstrate, and starting a fire inside city limits. Should be good for at least a year or maybe even a few thousand dollars in penalties especially if there is any booze or pot suspected on the person(s). Fact is that even though the actual burning might be protected by the 1st amendment, which I personally still doubt, there are other laws that are continually being broken along with the burning which are not enforced for whatever reason.
The flag is to represent the US and the dollars and change is supposed to represent money. If you destroy or deface money it is against the law but burning the flag is not. I guess that money is more important than the country that produced it