The basement (secret room) is under the detention room, as shown on the blueprint you get from Nate's locker.
Piece the comic book pieces together to get the locker combination, and get the gum from the guy at the Comics store. Go to the school and open the locker that's overflowing, and get the blueprint. Go to the Science Room and make a stink bomb (mix yellow and blue and heat it). Chew the gum and you get into detention. Use your stink bomb to get the teaher out of there, then click on the file cabinet to enter the basement. (There is a bell clapper hanging from the ceiling.)
when you get all of the comic pieces go to the person who needs the comic he will give you the gum then go to the science lab make a stink bomb then get out lab go to your stored items pick the gum when the teacher sends you into detention go to the file cabinet click on in and your in the basement.
out side the school
On Big Nate island the three girls stand on the playground to the right of the school .
The school is to the right of Main Street on Big Nate Island. The four main areas are Main Street, the school, the playground, and the lighthouse (Puffin Point).
near the playground
Poptropica
Up of the school.
It is visible in one of the animations for the school, but not as a separate document.
On the powerline
The bell is on top of the school in big nate island
chew it in school and go to detention
in the locker
you have to find the time capsule and that is how you save the school