Beware of the Haunted Toilet (Vintage Paperback) by Elaine Moore

The old toilet in the bathroom at the back of the class never acted up before. But this year it's flushing itself and making strange noises. The janitor says it's because of all the digging construction workers did over the summer to build the new school wing. Danny thinks that's probably right--but he wonders, if all that digging disturbed something else besides the plumbing.

When he and his friend Megan catch that something on video, Danny doesn't get it. Why would a ghost want to haunt a toilet?