If anybody is interested in learning about US boarding school options, a selection of the 10 most selective will be in London next week for their annual "Ten Schools" information fair. US boarding schools have multiple years of entry from 13-17 and cover the last 4 years of secondary school. The largest intake is at 13 (US 9th grade) and 14 (US 10th grade).
You can register to attend here: https://www.tenschools.org/page.cfm?id=352&verbose=2320