My Teacher Flunked the Planet by Coville, Bruce published by Aladdin Paperback

From the back cover: "Peter Thompson, a typical seventh-grader, finds himself touring the planet with his friends Susan Simmons & Duncan Dougal, and three aliens in disguise! Their mission? To file the final report that will determine Earth's future in the universe. As the clock ticks away the hours before their meeting in space, the tour becomes weirder and weirder. The three friends come face to face with a plague of poots and "Big Julie" - the weirdest alien yet! Meanwhile, Peter discovers a secret hidden for decades. Will his discovery save the Earth, or is it already too late to stop the aliens from destroying the planet?"