[the Obscure]

Actually, British English is probably more widely used - you've got the UK, Australia, Canada, South Africa, not to mention all the former British colonies that will be using the same spelling system. It would be really cool of these English teachers could acknowledge that both British and American English are correct, but since that probably isn't going to happen, you're going to have to cater to them. If your students want to know both systems, then by all means teach them both, but just make sure they know that they're going to need the British spelling for their tests.