The Invisible Fran (Franny K. Stein, Mad Scientist, Bk. 3)

Franny believes that nothing in the world compares to being a mad scientist and she's absolutely convinced that, if given the opportunity, every kid in her class would feel the same way. So Franny makes it her mission to bring out her classmates inner mad scientist. All she needs is a few open-minded students, a half-completed two-headed robot, and an invisibility potion... She's soon convinced three classmates that they know everything about science - the only thing is, they know NOTHING about science - and there's a lunatic robot running around to prove it...