Oh well, the teacher was way out of line. But then, we are dealing with the public school system where probably neither the teacher nor the students were ever taught that the swastika on its own is neither offensive nor inoffensive since it can be used and interpreted many different ways, depending on the intent of the user.
So in the end, only idiots are offended by one as it is only just a couple of lines---- it is the people often using them meant to inflict pain on others now in the modern usage which should offend people.
Unfortunately, there is very little real education going on in public schools these days, only INSTRUCTION.