Part 1: Teachers are like second parents
Like it or not, a teacher is like a second parent.
You'll find children coming home with new expressions, and peculiar ways of phrasing things. For example, one teacher would identify "parents" as "mommy-daddy", (spoken just like that, without an "and" in the middle.) The teacher is just teaching the way she knows how, and the children pick it up, like lint brushes.
Point is, children learn a lot from teachers, right down to the way they speak. Sometimes it's irritating, but generally not a big deal. I'll get to this point later, but you cannot control every influence.
Teachers should not teach religious viewpoints and so on. (There must be a guideline for what is and is not appropriate for a teacher to say in terms of passing judgements and talking about belief systems.) But invariably, when it comes to later (high school) classes like English literature, the teacher tends to bring out and teach their beliefs. This is just what these subjects are, and most teachers cannot help but explain things the way he/she sees it - ie interleaving their personal viewpoint with the teaching material.
Unavoidable, unless you're willing to bubble them in your own community (which works fine!).
There is a certain line in public schooling which must not be crossed however, and you can generally draw that line where it offends you. Say the teacher teaches "people with names ending in man are gonna be .. JEWISH!!!!" That might offend you, so you can go and complain to the principal and whatnot.
Part 2: Letting go
Most people won't agree with this section, but I think it is very truly important. Coming from an at least somewhat religious background, usually dictates maintaining certain views on certain things.
Favorite topics like gay marriage, or what Christmas is really about, or who's going to hell.
I strongly believe you have to let your child become who they want to be, as long as it's within certain acceptable parameters. (For example, you do not want them to become a drug addict and drop out of school. Reasonable.) But does your child have to hold your exact same views and exact same beliefs about the world? Or are they free to form their own? If you are very traditionally religious, the answer is "absolutely not they must follow in my footsteps or they will be doomed."
Think about that, and realize you cannot control every influence.
Have you ever played a game, like a space game controlling a massive ship, where the games controls do not respond well to too much micro-control? If you try to micromanage the ship's movement, you'll completely exhaust yourself, and the ship may crash. But if you give small, careful controls, then it will fly just fine, you just have to keep watching it and applying those small adjustments as it flies, or responding to emergencies if the ship goes really far off course.
So it's kind of like that. You cannot microcontrol human beings, you cannot have perception of their every influence, you cannot be there to tell them what is right and wrong for every input that reaches their ears and mind. You need to let go and let them become who they will be, but pay close attention to what's going on and exert corrective "forces" when necessary.
I will leave definition of the term "corrective forces" up to you.