The Mysterious Benedict Society by Trenton Lee Stewart

I read this book years ago so my recollection of the specifics are a bit foggy.  My eleven year old just read it and enjoyed it.
An ad in a newspaper lures many children to take odd tests. If they pass, they will be sent on a mission with mind-bending challenges.  Only 4 pass.  These 4, Constance, Kate, Reynie and Sticky, encounter some dangerous missions and must escape from an evil adult.
Very exciting!
-Borrowed from the library.