What's the Name of That Book??? discussion

In 5th grade, the teacher read this book out loud and my brain has been wracking for the title recently. I believe there was a double-decker red bus on the cover, and the title was written on a banner that swirled around the bus. The plot involved four kids who had passed some sort of test and they had to infiltrate a school/academy that was being run by whoever the bad guy was. There was also a brainwashing program that "relaxed" the students there and only some people got it (I remember the MC and another of the four kids getting it).

Random bits from the book that I remember:
-During the test, the MC eventually realized the test answers were hidden in later questions.
-One of the characters was actually a highly intelligent toddler(?)
-When the kids were infiltrating the school, on the first day the big bad was talking about how you had to live and mentioned that a lot. I remember specifically the MC realizing that "live" backwards was "evil"

The Mysterious Benedict Society. The cover that you remember looks like it is for the third book in the series.

Tathariell wrote: "The Mysterious Benedict Society. The cover that you remember looks like it is for the third book in the series."

THANK YOU SO MUCH! I thought it was a series but I wasn't completely sure, so thank you for confirming that also!

Yup came here to suggest that one too haha