That's interesting. Even Wiki has the school opening in 1957 which is consistent with my memory for reasons outside the record here. I think they in fact opened in the fall of 57 for the 57-58 academic year. But the only admitted one or maybe two classes--the Junior (11th grade 57-58) class of 1959; and maybe the Sophomore (10th grade 57-58) class of 1960 of which Stanley was a member.
But that would be consistent with your thesis if Stanley got in as a tenth grader (sophomore) in the fall of 57; would have had her there two years, sophomore and junior; and then she went to Chicago to work as an Au Pair and finish her High School there.
Here’s where it gets a bit tricky, one of the girls from the group image of Grade 10 we are still analysing, shows a girl who has been identified by her daughter, her birthday is October 27, 1942.
Her daughter maintains that her mother and Stanley Ann Dunham were in the Class of 1960.
If the date of that group image is correct, (and it’s dated by a caption, not something printed on the image itself) then it follows the Grade Ten group captioned as Mercer Island Students dates from 1958.
In other words, what we are looking at might be the first year of Mercer Island Summer 1957 to 1958.
The next grade, grade eleven, would be Summer 1958 to 1959.
And Grade twelve would be Summer 1959 to 1960.
The Grade nine group including the same identified girl shows her in a group with the same teacher who obviously belonged to one of those feeder schools - FOR THREE PREVIOUS YEARS.
So according to the above, any reports I have seen of Mercer taking students from 1956 may not be correct.