I'm going to suggest one more thing. That you call the principal back and let her know that you want to be sure you understand the policy, and that you also want to see what your daughter posted and to understand what was wrong with it. You can do this in a friendly, non-confrontational way, and say that you would like to be able to help her avoid problems in the future, but that you are having trouble understanding the policy and the violation. If you approach it as an issue you are also concerned about, and as a parent who wants her child to obey the rules, they should be happy to oblige. Kids have to learn that they have to comply even with rules that are "dumb" and "make no sense." Anyone who has ever tried to file a tax return knows that adults have to comply with dumb, senseless rules all the time.