Mandie and the Invisible Troublemaker

is the twenty-fourth book in the Mandie series.
==Book Description==
A new school term is beginning, and Mandie is saddened to learn the Misses Heathwood have put the school up for sale. Then a mysterious troublemaker begins pulling pranks--and making it look as though Mandie is to blame! It will take all Mandie's sleuthing skills to get to the bottom of it.
(Description from Mandie.com)