I read this because it was entered in SPFBO, and has a really good audio book.
I really enjoyed the story, and especially the main character, who is the only magister at magic school, who doesn't have magic.
If I read this as an adult book, I'd give it no more than 3,5 stars, as it lacks depth and just goes over scenes every so often. While the mystery itself isn't predictable, it's still often obvious what will happen, or why things happened, if you read quite some Fantasy books before.
This obviously didn't mean adults can't or shouldn't read it! If they, like me, enjoy YA as a nice bite between the grimdark or epic tomes, it's well worth the read.
If I read it as a YA though, it's a quirky, fun story with mysteries and section, that will suck you right in. I know I'd have loved it is I read it as a teen! I didn't want too much depth, and preferred a quick pace and lots of stuff "happening", without overly much descriptions or details.
It was a fun romp, and so I'll give it 4 stars, as long as it's in the right target audience hands!