I know two kids who did a full year exchange. One from a BS. Wasn’t sorry to miss sports. Figured out that she didn’t love living in a city after a year in a foreign city. Now at Midd, and applied very intentionally and well qualified to pursue language study. For her, an excellent choice.
Another from a public school. Came back after a year completely out of sync academically, no testing done, and had a very tough application process. Didn’t get into state flagship (suggesting that year didn’t translate well on her transcript). She had few good options but one worked well for her.
This is sooo dependent on the kid. I agree that this can be an amazing gap year. There is no way to truly learn a language except immersion.