Oh come on now.
First, who said this was a Japanese school? xD Not every anime is set in Japan. Think of it more as a meetup of various schools, since we'll all be wearing different things anyway.
And telling people not to wear one thing because you don't want them to... Isn't that a little controlling? It's like telling people not to wear Final Fantasy to a meetup. Of course, there will be less people doing this than that, but still.
Just ignore what the others are doing and do what you want :/ Skip the class if you want to, but don't try giving us a "Don't wear HP or I won't come" ultimatum.
Just to note, I'm not really a HP fan, I just support freedom to do whatever the hell you want that doesn't harm anyone ._.