That's a really good question and I think we should address it seriously.
To me, one of the best telltale signs of this happening for the worse is when there's a student concert and the students sound like discount copies of the teacher. I've seen this happen in a few conservatories myself, and it's always kind of sad to see. Granted this can happen for a number of reasons, not all of them necessarily bad. One is that the teacher is famous and people who already want to write like he does gravitate towards him, and two is that they may voluntarily also choose to write in that style because they like it. Those are two benign reasons I think are fine.
However, what you mention about manipulating is clearly got a negative connotation. I've also seen teachers basically throw students' ideas in the trash because "they've been done before," or "they're not interesting enough," or whatever. I've met composition students who were dissatisfied with their teachers often because they clashed on aesthetics, which is not uncommon.
Sadly, since becoming a composition teacher normally has no attached pedagogy study, a lot of people who become professors normally don't have that kind of background. Moreover, they may or may not be heavy on the ideology side of things, specially these days. I think that's also a potential point of friction. So, I always liked the approach my teacher had where he would not tell students what kind of language they should use, but he'd criticize them within their own parameters, if he needed to. For example: If you were writing something more traditional, he'd criticize it from a historical perspective.
He was very adamant that everyone should be free to select the musical language of their music on their own, for their own reasons, and I agree with this.
So, the question is, if it's unavoidable that a composition teacher may influence someone in their music, is this necessarily a bad thing? I'd argue that no. It's not always bad and specially if the relationship between teacher and student is built on respect and understanding. What teacher does isn't so much "corrections" as allow for discussion so the composer can see a different POV on an issue.
Well, my teacher's stance was that there focusing on uniqueness or "personal style" is not a worthwhile thing to do when you're studying. The idea is that you should write as much music as possible, hopefully try out all the stuff you're learning while you're at it. If there's such a thing as a style, it will happen to you as a result of your experience, not because you "tried" to do it. That kind of organic development, to him, was a lot more interesting and rewarding than just arbitrarily picking things because you want to stand out or "be different." Your music will represent your tastes and ideas regardless if you want it or not, so just trying to do it artificially seems like a waste of time in my opinion.
Additionally, people change over time, as do their tastes and ideas. It's obvious that this also affects their musical output and this is also why it should be an organic thing. Mind you this is also applicable if the teacher is trying to force their ideas and tastes on their students. This often doesn't work because the student will just write what they want to write anyway and simply not show the teacher, at which point studying composition becomes and exercise in futility and frustration.