Well, when a teacher is at work, they are a government employee. We have that separation between religion and state. While you are teaching, any expression of individuality, in this case religion, can be seen as a promotion of certain values by the state. Teachers have a very narrow SOP in this case, and rightly so, that's the way our system is set up. Do any of you know any teachers? If you think something like OH&S is bad, try reading the tomes they recieve yearly in some cases (at least in Nova Scotia). These manuals are constantly being updated, informing teachers how they are to teach, what they are to teach, etc.
You're free to be an individual on your own time, or at work I suppose if your employer allows it, but not as a government employee.