The first step in improving the essay would be to eliminate the first sentence, which is so vague as to be meaningless.
next, you need to have a thesis. Everything in the essay must be developed as support for a central thesis, but as it stands, the essay just does a vague riff on the term magical realism and then goes into the 3 standard examples for the 5 paragraph format.
Aristotle claims, correctly, that argument concerns those matter that are in doubt. To say that Skellig is written in the genre of magical realism is not a point with which most readers would disagree, and thus not a matter in doubt.
What sort of claim could be made about the work that is not universally accepted? Is there a way the three themes could be brought together as support for a single unified claim?
For a high school student, this is a fairly sophisticated essay -- but as you move towards university, you will find that simply listing three themes rather than having a coherent argument supported by examples and analysis becomes less acceptable, and so it might be a good idea to start thinking about more sophisticated structural approaches to writing.