The short answer: a thank-you note after a private school interview is graciousness, not strategy. Send one if it's true, short, warm, specific, and skip it without fear: no note ever sank an applicant, and no note ever rescued one either. The forums are wrong about this being secret leverage. It's a courtesy, courtesies are worth doing, and the version with one line from your child is the version worth sending.
Deflate the mythology first, because the internet has inflated a courtesy into a chess move. The note does not get weighed. Files are read whole, records, recommendations, the write-up, and a parent's thank-you email is not among the voices; it may get filed, smiled at, and genuinely appreciated, but appreciated and weighed are different verbs. Which is liberating news in both directions: nothing rides on it, so you can send one for the only reason that was ever good, because it's true.
What a good one looks like: three or four sentences, warm, and specific. The specific detail is the whole craft. "thank you for making Tuesday so comfortable; she hasn't stopped talking about the greenhouse" beats any paragraph of general gratitude, because specificity is how warmth proves it's real (the same rule your child's answers live by, it turns out). Within a week is nice; later is fine. Email is standard; handwritten is charming but earns nothing extra. Address the person who actually sat with your child, by name.
The child's line is the upgrade, with one strict condition: it has to be theirs. If your kid wants to add a sentence, "I liked hearing about the robotics room", that's the best version of this genre, in their words or their handwriting. What you must not do is assign it or ghostwrite it: an adult-composed "child" sentence is instantly audible, and it converts graciousness into strategy, which is the one way a thank-you note can actually go wrong. Invited, yes. Drafted for them, never.
The other ways it goes wrong, so you can walk past them: the second-application note, re-arguing the case, appending achievements, explaining the answer your child wishes back. The note is not an appendix to the file; the goes-badly piece covers the rare moment context is worth adding, and it's two sentences, not a brief. The follow-up campaign. One note, then done; checking in "just to reiterate interest" every two weeks reads as exactly the management admissions offices quietly track. And fishing: a thank-you that angles for feedback or signals stops being a thank-you in the first line.
So why send one at all, if it moves nothing? Three reasons that were always sufficient. Courtesy is its own reason. Someone spent a morning making your child comfortable, and that's a real thing to thank a person for. Modeling is another: your child watching the family close a big day with graciousness learns something no interview prep teaches. And the third is just true: admissions people are people, at the warm end of a long season. The note that expects nothing back is the one that lands, which is this entire site's thesis, applied to stationery.
Jennifer Hines spent twenty years as a Dean of Admission at Noble and Greenough School. Her free guide, What They're Really Asking, decodes fifteen interview questions from the other side of the table, and shows why the quiet kid is often the one who lands them best.