Tu versus Vous: The Never Ending Struggle

In French, like in nearly every other language, there is a formal and an informal way of addressing people: vous and tu respectively. I've been learning this language for more than a third of my life and I still sometimes have difficulty knowing when to use what. As an anglophone I'm not naturally wired with this distinction. As a basic rule, when in doubt, use vous, but it's not that simple. It's more polite, but it also creates a certain distance between you the the person you're addressing and can make you seem cold, and in certain situations (from what I can tell) it can be considered almost insulting.

Sometimes it's easy to feel it out:

Meet a random old lady on the street: Vous 

Talk to a small child: Tu

In a job interview: Vous

Among friends of friends at a party: Tu

But often you run into gray area, which is what I face at my schools. The principle vous me, I vous him, it's all good. But the teachers all tu each other, and they all tu me. I started off by vousing all of them, and only one of them specifically told me I could tu him and all the other 4th grade teachers. Problem: I deal with 17 or 18 different teachers a week, I have no clue who teaches fourth grade, and during that first week I didn't want to deal with remembering who I called what. At the same time I would have felt weird saying vous to some of the teachers and tu to the other ones. So I did what I probably shouldn't and kept right on vousing him and all the other teachers, even though it's starting to feel weird and way too formal, and it makes me something of an outsider. Today another teacher told me to tu her... shoot I don't remember which one it was.  

English, why you no prepare me for this?