If you have read any of Novik's other books such as Uprooted or Spinning Silver, then you know that she writes complex characters and gratifyingly complex plots. In this case you might try to imagine that Hogwarts meets The Maze Runner. That wouldn't begin to cover it, but would at least give you an inkling of the setting.
The Scholomance is a school for young magic users. Once you are inside, you do not come out
until unless you graduate. There are no teachers, no adults at all, so you may want to add a dollop of Lord of the Flies to your mental recipe. Since the presence of so many magic users draws the mals (maleficaria - "nasty things that routinely come after wizards looking for a meal"), students get plenty of practice on their offensive and defensive spells. They also form alliances, figure out who they are going to throw to the mals to make their own escape, etc.
The character El is the narrator. She is smart, snarky, and not good at social skills. One of the students finally explains that El makes others feel as if they are out in their best outfit without an umbrella and suddenly realize it is about to rain. That would put a damper on forming any friendships, you must admit. El's mother lives in a yurt in a commune in Wales and actually named her daughter Galadriel. El jokes that she is "the 'love me and despair' version" of Galadriel from the movies.
Without giving away any spoilers, let me just say that this book takes place in the junior year of El and her classmates. They deal with mal attacks in the hallways, library, shop class, even the dining hall. Those who come from large magical enclaves (yes, those are as ritzy and posh as they sound), have a head-start on the alliances and power brokering that will help them to survive past graduation, while El and the other who are loners or from smaller groups must be especially clever to stay alive.
The characters are from around the world - London, New York, Bangkok, Shanghai, and Dubai are just a few of the enclaves represented in the student body. The descriptions of the coursework, physical structure of the school, and how magic works are all well done and consistent. The maneuvering within and between the groups is like high school cliques on steroids. And along with the danger and deception, there are also really funny moments. An example would be the snack vending machine that randomly gives out items that are "usually aged, and sometimes inedibly ancient. Once I (El) got a military ration from World War I...Aadhya got ...a completely fresh salmon onigiri dated this very morning." How do you deal with something that randomly absurd?
If you enjoy stories of young wizards, especially if they are still learning their craft and trying to survive school (quite literally in this case), and figuring out friendships and relationships in general - then pick this up when it hits the shelves on September 29. You will be in for a treat.