If ANN’s episode titles are correct, it looks like…

(no fancy spoiler tags here, so below the fold it all goes)

...Rubi's story is squeezed down to a two or three-parter, with the remaining episodes focused on the enforcers.

• 1: New life: chapter 1.
• 2: Succubus: chapter 2.
• 3: Witch girl: chapter 5.
• 4: Goodbye: (original).
• 5: School swimsuit: chapter 3.
• 6: Newspaper club: chapter 4.
• 7: Snow woman: chapter 18/19.
• 8: Mathematics: back to chapter 12.
• 9: Summer vacation: chapter 13 (Rubi part 1).
• 10: Sunflower: chapter 14-17? (Rubi).
• 11: ?
• 12: ?
• 13: ?

In the manga, Rubi’s story takes a full five chapters, but that doesn’t make sense here. The end credits have the enforcers from chapters 8-11, and even if they’re collapsed down to a two-parter, the only things left to use would be 6, 7, and 39. Chapters 20-28 are pretty much one story arc, as are 29-34, and the school festival in 35-38 requires chapter 6 to make sense. Chapter 40 is more of a new-season intro, since it was written as a standalone introduction to go with the change in magazines.

So, the choices are:

1. 3 for Rubi, 2 for the enforcers.
2. 2 for Rubi, 3 for the enforcers.
3. 2 for Rubi, 2 for the enforcers, 1 for chapter 7.

I don’t realistically see them animating chapter 6, because someone would have noticed her in the credits, and if they aren’t doing the school festival, she’s just another monster-of-the-week. If they get a second season and decide to use her, her intro can be dropped in pretty much anywhere.

You could make a pretty decent second season with chapters 6 and 29-40. You could make a dark second season with 6 and 20-34. I’m not sure you could get something coherent if you tried to include significant elements from both 20-28 and 35-39.