That's splitting hairs, isn,t it? It's a tiny island separated from downtown Seattle by a bridge, and it's considered part of the Seattle metro area.
If Robeson had taught there, it would certainly have been remembered, and celebrated, by Seattle historians. According to his bio, he underwent major protrate surgery in 1955, and became increasingly mentally disturbed. When his passport was restored by a Supreme Court ruling, he gave concerts in the both the Soviet Union and Carnegie Hall in 1958, then moved to England sometime before 1960. There's no evidence of, or rationale for, his teaching at Mercer High School during the years Ann Dunham attended.
You are right. I made the whole thing up. Who would think a Moscow appointed Committee Member of the CPUSA would be teaching at a Marxist High School with other teachers that were members of CPUSA.
And if you really concerned about the ‘splitting hairs’ of the location of Mercer Island and your quote that means nothing, you would do searches on my prior posts on Paul Robeson to find the cites.