My US History II teacher in Junior year was an outspoken Conservative, and I can say that I have never had a better history lesson taught to me in my life than that year. I am not saying that it really matters what political beliefs you hold, but Conservative teachers tend to tell kids things more straight up than Liberal teachers who have shown to me in the past the inability to stop making things PC.
To talk about the Mexican-American War, I would have loved to hear more about it since it costs so many American lives to win anyway. I am not ashamed of it, the US was merely taking part in something every nation with aims of power in the future did. You know what's ironic, because we actually annexed our conquered land into our actual country rather than make all conquered subjects second or third class citizens, we probably have one of the most successful and long-standing empires of the 20th century. (Although I believe the British might beat us at that)
Anyway, I think that our teachers should just tell us things strait up and not make the world seem like all Peaches and Cream. Life is tough, you live hard and fast and then die. Accept it, humans do despicable things but I have this belief that in the end everything has happened for a reason. (I mean a positive reason that affects the future)