I have no experience with Milton, but from my experience elsewhere math placement is based on the placement test. Most 9th graders place into Geometry if they had Algebra I in 8th grade.
In general, 9th graders have very little flexibility in schedule. They take the 9th grade English, history, and science course, and the math and foreign language course in which they were placed. If the school allows a 6th course as an elective, there may be opportunity to change in consultation with the adviser. If your question is changing a course in which you were placed, generally, you can’t - at least not initially. 99% of the time, the placement is correct. If the placement was wrong, they will generally adjust very early in the term.