I read Harry Potter when I was a kid so my memory may not be accurate but in retrospect the timeline of the first wizarding war is hilarious. My impression from the books was that when Harry's parents were in school everything was peachy keen and shit went south after they left school and joined Dumbledore's Army, but ultimately it's revealed they died when they were like 21. So does that mean Voldemort gathered an army, rose to power, did all the heinous shit he supposedly did, and was defeated in four years? I guess it's plausible but it seems like J. K. never really thought out the timeline of Voldemort's initial rise at all.
According to
the wiki, the first wizarding war took place in the 1970s, when Harry's parents (both born in 1960) were at school. Their entire secondary education happens during this war. It does not surprise me in the least that flashbacks to the Marauders era fail address this. I'm a little disappointed that the wiki doesn't even give much of a description of what happened during those
11 years of total war. If it was enough to justify a mass celebration when it finally ended, it must have been really bad. But I shouldn't be surprised when my beloved fantasy novels skimp on details.
Voldemort also had a decades-long rise to power in the 40s, 50s and 60s, on account of all the "50 years ago" stuff in book 2, leaving a decades-long gap in the dark lord's CV that Rowling later had to fill with
evil stuff. I don't know why Tom Riddle needed 25 years to corrupt his soul and become Voldemort when most trannies manage it in 2. But then again, maths was never JKR's strong suit.
The closest thing Hogwarts has to a maths education is an optional class called arithmancy. Hermione takes this class, but we never get to find out what it involves because JKR doesn't know. She's so innumerate that she doesn't know how many students there are in Hogwarts. She once
said in an interview there were 1000, but this number divided by 4 houses, 7 school years and 2 genders
(REEEEEEE!!) would imply around 18 Gryffindor boys in Harry's year, even though it's confirmed on-page to be 5 (Ron, Neville, Dean, Seamus, and Harry himself). Extrapolating from this sample, we get a school of just 280 students.
According to some fan theories, this is because the birth rate fell dramatically in the wizarding war. There is a reason why fan theories are such a recurring part of this series' lore, and it's this:
The best – and worst – thing about HP lore is that details are so thin on the ground, it's actually very easy to explain stuff away, as Rowling herself has done. Like for example, the plot of book 7 required Harry to be an adult even though, being born on the 31st of July (same birthday as Rowling), he turns 18 at the
end of his final school year. So she just made it so wizards come of age at 17 and explained this in book 6. No previously established fact contradicts this. And no mention of his friends having important birthdays earlier that same book, but there almost didn't need to be. The protagonist-centred morality extends to never remembering his friends' birthdays.