I found the last drum ^_^.
You have to go through the library, then through that tiny little hall with that picture of the kitty graveyard (although, I wonder why the secret society has a picture of that old lady's backyard....) and through the other door that leads you to outside. go down that little ramp. You will NOT see a ! Instead, look at the touch screen at the bottom right corner until you see an X. Yay!
I can finally go get some rest. *sighs*