I was oversimplifying... Of course there's going to be some correct way to spell things. What you have to realize is that there isn't one set way to spell all the Names since there are various ways to pronounce it; you can use the Michael example above. So it all depends on the intended pronounciation you're looking for and having the correct set of characters to represent those sounds.

Also, your Japanese teacher is not absolute... the Japanese language is complicated as it is, that I'm sure that even the teachers are still learning some things about the language. I know I've seen a lot of discrepancies between teachers and the romanization of the small "u" character.

For example, some would argue that "Tookyoo" is the correct romanization while others would argue it's "Toukyou".