Bryan, you really seem stuck on this point, so here's some helpful info for you.
A teacher, in the performance of his duties during school hours, acts as an agent of government and is required to abide by the restrictions placed on government as specified in the Constitution. If he doesn't, he's breaking the law.
Violation of the First Amendment's requirements is, in fact, lawbreaking, just as if a judge showed gender bias in the performance of his duties, or if a DMV worker refused to wait on a Hispanic citizen. Those actions by government employees are illegal, simply because they violate the terms of the First and Fourteenth Amendments. So yes, an individual certainly CAN break those laws.
But then, such an understanding doesn't advance your "stop the secularists" agenda, so I can see why you'd resist it. Doesn't change the facts, though; a teacher preaching during school hours is automatically and inherently breaking the law, just as if he'd automatically flunked all his Muslim or black students.