Thanks for doing that, I found myself wishing they had put error bars on when I saw that graph but was too lazy to do it myself.

With respect to the statistical power issue, it may be they just didn't have enough students. Unlike your typical clinical trial or scientific study where you're recruiting people, I got the impression this was from an existing pool of students for a specific school. Once you go beyond this, the design gets tricky since there are practical issues to how big a class can be. Then you need more teachers, and you need to normalize for per-teacher variability. Not saying a study like that wouldn't be great, but it would be complex to get right.