I’m sorry, I just couldn’t read through the whole thread, so I apologize if I’m repeating something someone else said.
FIRST and most important: Val and Sal are not determined until well after acceptances are out. Nobody gets to say that they’re valedictorian on their application. So there goes the OP’s whole theory right there.
Second, the OP is confusing cause and effect. Is it likely that Dartmouth has a strong preference for people who are number 1 in their class as of December of their senior year (or possibly the end of their junior year, I don’t know how often high schools recalculate) vs people who are, say, number 3 or number 5? Think it through. Why should they? They know that at many schools, those people will be very close. Their GPA’s may only differ by hundredths of points. The difference between a 4.21 and a 4.23 is no more interesting or informative than the difference between a 790 and an 800. Lots of vals will be rejected, just as lots of people with 1600 SAT’s will be rejected. The person who is number 3 at application time may be 1 or 2 at graduation. Dartmouth would have a very hard time selecting for vals and sals. So, does Dartmouth have a lot of vals and sals because they select for them specifically? No. Dartmouth, which admits holistically, seeks out many different factors, among which are high grades. A school with very competitive admissions with lots of applicants with very high grades will, AS A RESULT, find itself admitting lots of vals and sals. Then, they use those numbers for marketing purposes. They don’t have to select for them, they naturally get them.
Third, schools say class rank is important because they want to see that you are doing very well against your most immediate peers, to the extent the information is available. It’s not the same thing as saying they care whether you’re 1st or 4th in a class of 300.