I'm idly flicking through TVTropes, as is my wont, and on the page
Compulsary School Age (about characters who
look like they're in their teens having to go to school, even if they're Really Seven Hundred Years Old), I read this:
*The 2002 tv series
That Was Then and
Do Over both featured protagonists in their 30s who were
reliving their high school years.
Aha!
Now I know why the This Time Round
High School AU created by
scarfman is called
"Then Do That Over"!