I loved the first one, enjoyed the second one but this one was just bad.
There is no structure to the story, it's incredibly repetitive, and the whole climax is a horrible mess.
In regards to structure, this is actually something I enjoyed about the first two books. I couldn't predict the classic story beats which usually happen like clockwork in most books. But the pacing worked really well regardless. There always was some sense of progress and some kind of immediate conflict. In this one, we are basically stuck throughout the entire book.

It almost seems like, now that the author is done with the worldbuilding foundations, she doesn't know what to do. It kind of feels like bad second book syndrome. (But it is the third one, I know.)

The biggest problem is the classic issue of how do you create tense situations if your MC is too powerful already. Sadly the author has no answer. So we ended up with a mix of tstl, tstl, tstl, and a few outright contradictions of common sense and science.
The TSTL is really bad. And it came out of nowhere. The MC was a bit obtuse at times in earlier books but I can accept that especially considering how stressful her situation always is. In this one, her IQ suddenly drops to room temperature on the authors' whim.
If you mix your magic with regular science you can do literally anything with your magic. But you can NOT adjust physics the same way. If it is a planned change that intertwines with the magic it can sometimes work if done carefully. But that is not what happens in this book. The author constantly just makes shit up on the spot as an excuse for whatever she wants to happen. She does this with all aspects of the story but it hurts particularly badly around scientific stuff.

A few questionable things about common sense already popped up mostly in the second book but those were all minor and easy to ignore. Not so in this one.

I very much feel like this book has been written top to bottom in one go. No revisiting of earlier writing or reworking scenes that didn't quite work at all. Maybe there was some extremely tight deadline involved?
But it doesn't seem like the editing suffered in a similar manner. It is still spotty just as in the first two books but it didn't get worse.

While I am somewhat mad at a lot of messed-up details I frankly am not interested in how the story will progress either.
Considering how strong the main cast is it is quite sad how little the author did with it all. I can see what she is trying to do and I would enjoy that, I am sure, but the execution is just really bad.
( )

So the second book introduced (more) complications into Cori's life. This one plays out some of them...and introduces more. Of course. Little things like someone's trying to kill her, more important things like new teachers…I give up. I need to just read this series through (but there's only two more books out so far!). ( )