Found this one in the progress of writing the question, but didn't want to waste my efforts, so posting with a self-answer.
It was a film that my wife and I watched on DVD about 5-10 years ago featuring a young girl who is sent to a remote boarding school set by some woods. Upon arrival to the school, she takes a placement test, which she is told she got high marks upon. I seem to remember there being something hinky about the test, with the questions not making sense and her marking it more or less randomly, with some speculation in-universe that the test is a sham, a way for the teachers to pick their students while claiming a lack of bias. There is a legend about witches in the school's history, tied to the disappearance of students, and I believe the climax involved the girl finding out that the teachers are, or believe themselves to be, witches, and that the test was actually a test of their "potential", although I don't recall if it was supposed to indicate potential to learn magic, that they were reincarnations of the legendary witches, or something else.