First of all, definitely send one, within 24 hours. Optimally not like 30 minutes after the interview; it seems more sincere if you have done some reflection. But no later than 24 hours.
As for the content. Show you were listening. Highlight anything they said that left you with a 1) strong positive impression of the school, and 2) anything you felt particularly resonated with your values or strengths.
Also, note something you might have forgotten that you think is important, after reflecting on the conversation. A datapoint you think would help establish fit/rapport.
I encourage my applicants to be as specific as possible, rather than make general statements or impression. Like when you mentioned XYZ I thought to myself XYZ. More so than “it was great to hear that the culture there is collaborative.” zzzzz
When it feels natural, quote or paraphrase them to show that you have the capability to listen to other people and their experiences. This makes you look like a more attractive learning team member, and nothing is worse than having a dkhead self-centered learning team member. A lot of these alumni are, in part, out to protect the experiences of their future alumni.
And finally, many people ask me if they do not reply to their thank you note, does that mean they are dinged? Or if they do, does that mean they are in?
Well, if they send back something like, “rooting for you!” it might be unlikely they submitted negative feedback unless this person is a straight up sociopath. But I dont want to scare thank you senders who dont get a response.
Many interviewers feel its important to be proper: avoid any illusion of bias or “hinting.” Writing back might not seem proper to them. They are afraid of attracting an applicant-stalker. Who knows.
But, keep in mind they might smile to themselves when receiving your note. In sum, write the note. It’s a way to solidify your value proposition in their mind and to show them you “get it” when it comes to the business world and being civilized.
PS: For more guidance, research HBS post-interview reflection samples.
Hope that helps. Good luck!