A History Mystery!

Marc Miller, Class of 1993 was written to us with an inquiry. Marc writes; I'm researching my grandparent's childhood at the moment: Abram and Esther Wessels. My mystery question is below.

Abram Wessels, Class of 1939 and beloved SHS Teacher!
The mystery I have is from when they graduated high school.
 
They are listed as Seniors in the 1939 Yearbook. There is a newspaper clipping from June 23, 1939 that says they got their diplomas that week.
 
My grandfather played basketball and baseball as well as acted in plays in high school and those things got mentioned in the newspapers.
 
Yes, there are newspaper clippings into 1940 that mention his games and box scores. As well as a game played at Madison Square Garden.
 
Why in the 1930s and 1940s someone would get their diploma in what would seem to be their Junior year, but then go to school for one more year?
 
My grandfather was born in February of 1927, so he would have been 17 in June of 1939.
 
Why would the 1939 yearbook call them seniors if they were coming back for the 1939-1940 school year?
 
Why would they get their diplomas in June 1939, but come back to school again for another year?
 
Interestingly, the article in the April 26, 1940 clipping says that was his last game as he was leaving to go to work in the city. That may be a clue. That seems to indicate he wasn’t coming back for the rest of the baseball season (which would go at least into May).
 
Thanks for any insights everyone might have on my mystery!
-Marc Miller, Class of 1993