I would suggest that they usually do not. Winning the slate is still a fairly objective process based on your objective parts of the application. . For the prep schools, the candidate was most likely not qualified the first time in the academic portion so they would have a real hard time raising test scores and overcoming the high school record. The prep school candidates just need to complete the program and they will be offered a slot as an additional appointee. They do not need to win a slate.
Civilian Prep is a bit different and It just depends. If the candidate just missed the year before and strengthens the application they will have a good chance to win. If they were a marginal candidate the year before, the semester of prep school will not overcome the deficiencies.