Back in the 1980's Marvel released a book called the "Marvel Saga". It was a book that placed several stories of different characters in chronological order during the silver age.
You may not find the reference in an actual FF book, but in the Marvel Saga Series, (I assume it was written by Peter Sanderson) it was noted that Johnny received his high school diploma in secret when the team lost their powers, for safety reasons. The FF went underground until Reed could create devices to duplicate their lost powers (like the Thing exo-suit). Also around that time, Balder came to Earth to defend Jane Foster from the Enchantress & Executioner. The Frightful Four was circling the Baxter Building when Balder arrived. He looked like a large ball of flame which caused the evil FF to think it was Johnny (and fled). This happened before Doom invaded the Baxter Building.
Johnny getting his diploma might be considered "behind the scenes" of FF #38-#39.
I wish I could remember which issue of the Marvel Saga stated that. It could be issue #18, or #19.
https://comicvine.gamespot.com/the-m...vel/4050-3490/