Once, in [no article] history class,...

I did check about 'a' - https://www.google.com/search?q=%22in%20a%20history%20class%22&tbm=bks&lr=lang_en
Is this case special in any way? After all, it was *some* class.

It's hard to explain, but if you use "once", you go into storytelling mode and are referring to the class by name as a class you attend regularly as part of your curriculum. I can't think of a situation where you'd start a story with "once" and use "a history class."

OK, thanks