I am looking for a word or phrase to describe students at a K-12 school who have attended this same school the whole time from kindergarten through 12th grade. The first of these students will be graduating from our school soon, and referring to them as "long-term students" or "early adopters" doesn't work well to get the meaning across. Thank you for any help you can give.
"Lifer" is the common term used at many K-12 private schools in Virginia. Based on a quick check of available yearbooks, I can confirm it has been in use since the 1970's. I'd really never thought of it having a negative connotation before reading some of the comments here. The kids are proud to be "lifers" and do not think it a disparaging term.
A quick search on the website of The Collegiate School in Richmond, Virginia yielded many examples, including this description of an alumna receiving an award: "Cathy is a Collegiate lifer. Her daughters Carolyn ‘12, Frances ’14, and Julia ’17 are lifers as well." https://www.collegiate-va.org/page/news-detail?pk=824643
tchrist's comment on "lifer" got me thinking, would "lifer student" work? Granted that wording seems to be very uncommon and would likely require further explanation, but I did Google it and surprisingly a result came up that describes the same kind of students in your post: http://www.nuevaschool.org/news/upper-school-news/1869-lifer-dinner-2017
...students at a K-12 school who have attended this same school the whole time from kindergarten through 12th grade...
Unfortunately there simply is not such a term. (This is often the correct answer on SWRs; there's nothing wrong with a null result.)
Commentor @Riccati gave an excellent suggestion...
I doubt we'll find something better.
"Graduating this year we have 202 students, and I am proud to announce that 91 of them were 13-year students here at East High! Go Wildcats!"