Malone
Interesting that you don't like "madam", and I agree it is not the equivalent of "mister". I teach an after-school class, and the problem is how to address adults when you don't know their names. If a teacher comes in, I use "Sir" or Miss (whatever her age). This seems to be universal in schools here.
If it is a parent, I use "Sir" or "Madam". I agree with you that it does sound old-fashioned but I cant think of anything better. The Head is a woman, as was the previous one. I address her as "ma'am" in class and in one-to-one meetings. Both ladies seem happy with this.