No no, okay, how it works in there are those seven categories, and the seven themes inside each category.
If you want to write the high school AU, you either need to claim the entire profession category, and write six more stories using each of the other themes once (one story with "college student" one with "sports team" etc) OR, you need to claim one theme from each of the other categories (so, if you wanted "high school" from profession, you'd then need to pick six more themes, like "pirates" from cosplay and "play one on tv" from meta, etc)
the minimum number of fics you'll end up writing, this way, is seven. Make sense?