Casper Tock and the Everdark Wings

Casper Tock spends his days hiding from bullies at the private school he attends on scholarship thanks to his parents' jobs there.  He's so busy guarding his heart that he doesn't have friends.  One day while running from bullies, he accidentally discovers a portal to another world.

Rumblestar is one of the unmapped sky kingdoms that controls the weather on earth.  The first person Casper meets is a girl named Utterly who is convinced he is a demon.  There have been problems in the unmapped kingdoms leading to tornadoes and other extreme weather on earth, and Utterly thinks Casper is responsible.  

Utterly has always felt like an outcast.  The kids at school openly mock her, and most people think she is a failure.  She thinks this is her opportunity to prove her worth, but by the time she realizes Casper may be innocent, he's in more trouble than ever.  Now she will have to rescue him, and these two social outcasts will go on the adventure of their lives.

This series opener from Abi Elphinstone is a good fit for middle grade fantasy fans.