These are both great books, American classics, and everyone should read them.
I think high school kids are old enough to read them. But they require context. They require that the history of racism in America be acknowledged, discussed, and that the instructor contrast the social conditions depicted in these books against race relations today, and also against how they "should" be (because racism isn't solved yet).
I think this last part might be the problem. To teach these books in a manner respecting all communities requires taking a clear anti-racist moral position. Because kids will run across racist language in these books, and if it isn't covered in a normative context then what kids are going to get is "this is how things were ... " -- shrug -- and some kids are going to learn that racism is okay.
I'm sure that any high school teacher realizes this. But I bet that a lot of high school teachers and especially administrators are afraid to take a clear moral position on racism in the classroom out of fear that some parents will revolt, or that they'll end up in a Fox News segment on public school indoctrination.
They've probably been avoiding it for awhile, out of respect for "both sides." But this weak approach is worse than not teaching the books at all. So they've decided it's easier to just avoid the issue entirely.