Is it common for an American to use British English spelling?

Well, I'm fourteen and I have lived in Germany for most of my life, though I was born and have lived in the United for some years. I regularly read books written in American English, but when I write my own, I use the "variant" spelling of words. For example, instead of "favorite" (American) I spell it "favourite" and instead of "realize" I spell it "realise". It's getting annoying because my Microsoft Word keeps correcting my spelling, and I've been a great speller all of my life, it's just the words that are spelled differently (like grey in England and gray in America) that get me. Is there a way I can correct this before I start using British English in school? It's getting hard, and particularly annoying because I have to erase every word that has a UK spelling and rewrite it in American English, and it doesn't help that I have to write in cursive.