Repeating a year in boarding school?

So, if my 14 year old 9th grader ends up in a highly competitive boarding school next year, is she destined to be much younger than other incoming 9th graders?
Obviously she doesn’t intend to repeat 8th grade and definitely plans to go to high school somewhere next year. Are there some families going into this who intentionally plan that their kid will go to 9th grade at one school and then transfer to a boarding school the following year as a repeat 9th grader?