Through the Portal (Through the Portal #1)
Alternate-cover editions can be found here and here
When Jem and Oliver accidentally fall through a portal to another world just before their first year of high school, they quickly discover that all is not well here. The first person they meet, a creepy old man named Atychis, almost gets them killed by a ferocious, fire-breathing dragon. They're only narrowly saved when Sierra, a shy farm girl from a nearby town, uses illegal magic to help them escape. Allowed to stay with her family while they try to figure out a way back home, Jem and Oliver begin to learn of magic and the Regime that is oppressing it.
It isn't until the Regime kills a woman that the three kids realize they have to do something to stop the Regime from taking over completely. After being framed for a crime they didn't commit and banished from the town, Jem, Oliver, and Sierra take off on an adventure across this strange world in an attempt to defeat the Regime. New creatures and new kinds of magic are around every corner, but so are dangers that could have them wishing they were back safe at home.
Through the Portal is the first book in a planned trilogy.
Approx. 104,000 words