Another mystery solved.

[daibhidc]

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"!