Aliens Ate My Homework (Alien Adventures #1) by Bruce Coville - Vintage Paperback

IT'S THE WEIRDEST ALIEN INVASION EVER!

"I cannot tell a lie," says Rod Allbright. And it's the truth. Ask him a question and he's bound to give you an honest answer. Which is why, when his teacher asks what happened to last night's math assignment, Rod has to give the only answer he can: "Aliens ate my homework, Miss Maloney!"

Of course, no one believes Rod this time, so they don't bother to ask him why the aliens are here. It's just as well, since he is sworn to silence about their secret mission and the fact that he has been drafted to help them!