The Time Hackers, by Gary Paulsen, 2005

Dorso's getting tired of holographic artifacts, such as rotting corpses, showing up in his locker. When Custer and Beethoven appear and seem to see Dorso, defying all the tenets of the grandfather rule of the paradox of time, he realizes something's wrong with his time travel circuitry, and so long as he has his laptop with him, he's liable to be swept into a critical juncture of history, and right in the line of fire. As with most time travel fiction, the attempt to explain the metaphysical premise grows convoluted and wearisome. However, this novel packs a lot of action and humor into remarkably few pages. Dorso's friend Frank singlehandedly justifies the paper the novel is printed on. A fun, quick read; recommended.