Had I been Umbridge, this would have been like a big warning sign---a firebell in the night, to awaken me and fill me with terror. A "white mutiny"---where subordinates do only what they're directly ordered to do, as exactly as possible---is one of the most insidious forms of rebellion I can think of. After all, what can one do about it? Punish the mutineers---for obeying orders as precisely as possible? Voldemort could probably have broken the teachers" resistance, but Umbridge couldn't use the sort of methods he would use without destroying the moral basis of her power. Not to mention, she didn't have the sand for it. Or so I thought.