Marchalonis's timeline suggests a correspondence - emphasis moved away from women's colleges (and thus the friendships, romantic or otherwise, of the students) toward co-ed universities - and the stories that were still set at women's colleges moved their focus from the delightful college experience to how *unnatural* and *unhealthy* it was to have all those girls cooped up without any boys.

I recently read an English novel from 1917 that's set up around this very theme - it seems to me that overall the 1910s were the years when the sexologists' writings about homosexuality began to seep through to the general public and make them view with suspicion romantic friendships that had earlier seemed sweet and innocent and even constructive.

But Blyton was still writing school girls getting harmless crushes on each other in the forties, so clearly it didn't die all at once.