It all depends upon your relationship with the teacher. If you know them well, and you know they wouldn't mind, or you know enough to know it won't offend them, then no, it is not rude.
But, since you asked the question on the Internet, you are indicating you do not have this kind of relationship. So, you are taking a chance. Some will be offended, some won't care, some won't even read it. You just don't know the person well enough.
Because of that and because of the way people are so easily offended, you are being rude, very rude, to send it. You are asking for trouble, regardless.
If you are that worried about it, as other(s) suggested, bring it up. Perhaps,
"I looked at that textbook and read the teaching suggestions. I would
not want to be you. That looks hard. I wouldn't want your job for nothin'(g)."
Or just ask,
"Teacher, Are we gonna get to study x this semester? I hope so. I
really wanted to learn that."
That is true and you won't look bad asking, more than likely.
You will still risk offending someone because you are going to have to work very hard not to sound condescending or concerned. People are not (too) dumb. They know when you are not sincere. It will be difficult for you to hide this. Or, maybe you're good at this sort of thing. Who knows?
But guess what, no matter how hard you try not too, you will offend some people at some time or another. People get their feelings hurt easily and are insecure. Some people you simply cannot win with no matter what. They read into things ranging from email, Facebook posts, and phone conversations. I know this firsthand because I'm a fallen human.