The Principal's on the Roof (Fletcher Mysteries)

WARNING: Reading May Be Hazardous To Your Health
The principal of Jill and Gwen's school promised to spend the morning on the roof if the kids read one thousand books during their reading marathon -- and they did! Now he's on the roof, and he's going to read an original science fiction story Gwen wrote.
But he could fall off the roof is he doesn't stop sneezing. Is it a coincidence that Gwen's story is about aliens who live on roofs and make humans sneeze? Fletcher knows that Gwen would never pull a dangerous practical joke just to make her story come try. But can he prove it before Gwen's in big trouble?