Reading helps you learn grammar, and when you understand grammar, your writing is better. Being a good writer makes school easier, and it also helps you communicate clearly, which is an essential life skill. No matter what your path in life is, you will need to communicate with other people.
Reading books, especially fiction, exercises your imagination. Just like your muscles, your imagination gets stronger the more it is used. The stronger your imagination is, the more creative you become. And the more creative you are, the better you are at solving all kinds of problems.
The more you read, the better at it you become. Like anything, it takes practice! If you read often, you will be able to read faster and understand written language better than if you hardly ever read. Because reading is an essential part of all of your classes (yes, even math!), school becomes easier as you become better at reading.
Reading books that put you in someone else's shoes—fiction and memoirs, for example—helps you see the world from perspectives other than your own. Being able to see the world from another person's point of view is called empathy. Building empathy helps you understand the people around you better, and makes you a more compassionate person.
If you love to read, you can teach yourself almost anything. History, law, physics, plumbing, poetry... no subject is off limits to a person who loves to read. And the more you learn, the better you can understand the world around you, and the more you can think for yourself. With knowledge, you become empowered.