Peter getting his powers at age 15 was a retcon. In Amazing Spider-Man #8 (
Oct 1963), Peter is a high school senior. He doesn't graduate until #28 (
June 1965). From what I recall, there isn't anything to indicate that Peter was in a lower grade in AF #15 or ASM #1-7.
Peter begins college in Amazing Spider-Man #31-33, a story that spans four days. In #34 Aunt May is released from hospital, so only a short span of times has passed, Jonah also gets a new secretary. In #35 Peter finds out that Betty quit her job and meets Jonah's new secretary for the first time. In #36 Peter is still in his freshman year. In #38, Ditko's final issue, Betty's disappearing act is still treated like a relatively recent event.