Seventh Grade vs. the Galaxy — Joshua S. Levy

PSS 118 is just your typical school -- except that it's a rickety old spaceship orbiting Jupiter. When the school is mysteriously attacked, thirteen-year-old Jack receives a cryptic message from his father (the school's recently-fired-for-tinkering-with-the-ship science teacher). Amidst the chaos, Jack discovers that his dad has built humanity's first light-speed engine -- and given Jack control of it. To save the ship, Jack catapults it hundreds of light-years away and right into the clutches of the first aliens humans have ever seen. School hasn't just gotten out: it's gone clear across the galaxy. And now it's up to Jack and his friends to get everyone home.