I do not know all that much about private boys' schools in that time and place, but I am wondering whether the school followed a British system that had boys starting much younger than ninth grade. If you read enough English novels, you can see that boys were sent away to boarding school at a very early age. The designations might not be anything like those we have today.
The other aspect of what seems to be a discrepancy could also be the result of the war. There is evidence of acceleration, for example, as Gene says,
"...this was the Summer Session, just established to keep up with the pace of the war" (15).
If 17-year-olds could enlist, which I think they could, the summer courses might have kicked Gene and his cohort into senior status. They could then have completed their educations and still enlist as quickly as possible. When they graduated, the outcome of the war was not so clear, and we needed as many men as quickly as we could get them.