Is it norm to write a thank you note after a grad school interview? I consider interviews are just part of application process for which applicants pay a sizable fee. Do you still write a thank you? What is an established protocol?
I can’t think of a scenario where it isn’t appropriate to write a thank you note. So, yes, write the thank you note. The sooner the better.
Within 24 hours. Just a short email thanking the professors who interviewed her and including a specific point or two about why the kid liked the program/lab/department etc. and why the kid thinks she would be a good fit.
(ETA - Autocorrect, are you still asleep?! )
As @BunsenBurner points out with her suggestions, a well worded thank you note isn’t just a way to say thanks but also a way to restate your interest/sell yourself another time.
What if you don’t want to encourage them into thinking you are coming? You don’t want to be rude but you are not coming?
So then you are asking if you should write a “Thank You but No Thank You” note? Let them know you are not interested. Why waste everyone’s time or take a spot away from another applicant?
Igloo, it is up to the kid to skip the note writing. I do not recommend it, because if the program is super selective, any bit that makes an applicant memorable (in a good sense) can tip the scale in her favor.
Her goal is to get an offer. Then she will be thinking about rejecting it or not. It is a game everyone plays, and it is not “rude.”
Interesting, I didn’t know most people write a thank-you note. It feels a little like sucking up but I am clueless.
For a Ph.D. program interview? I think a short thank you email is great. I’d guess about 1/2 the people send one.
Personally, I think a short note makes sense just out of simple politeness from one human being to another. I don’t think it’s appropriate to think of an interview as “services paid for”.
On top of that, this isn’t an alumni interview for undergrad admissions, which are often almost meaningless. These are the decision makers you’re meeting with.
Also, some of the people you’re meeting with might be a potential Ph.D. advisor or recommendation letter writer. Remember that academia is a very small world, and this is Networking 101. In academia, a lot of the feedback is conveyed through word of mouth. Even if you don’t do your Ph.D. at this program, there’s a good chance you’ll see these same people again over the next decades when you’re applying for post-docs or assistant professorships, or at conferences. One of them might even be asked to write a letter for your tenure file 
How about open houses? Do you also send a thank-you email after attending? I guess it also applies to a Skype interview.
I always had the idea that any place which would consider my not sending a thank you note as part of my application is a place I probably wouldn’t like anyway.
I would think it doesn’t hurt though. Depends on how far you take it. I have gotten them from people who sent one to each and every person they met that day. At some point, I think you need to cut it off to keep them at least somewhat meaningful.
my thank you notes are carefully selected cards that I hand write over and over again until they are legible. then I fuss over printing the envelope address with my home printer and decide whether to do the same with my return address or use a stick-on address label. all is always exacerbated due to the fact that my carefully selected cards are never standard sized #10 envelopes. I save the email for #2 or #3 post interview, which is a toss up either call or email, sometimes I switch them around. never interviewed for grad school tho!