You first have to get in the one locker all of the papers knock you back. Then you have to go to the giant pile of papers in front of the locker. You will find a map leading to the detention room secret door. Then you can get detention, set off the stink bomb, and go in the secret room.
The entrance to the secret room under the detention room is in the filing cabinet.
Find the bell clapper in the secret room under the detention room, and repair the bell. (see the related question)
Big Nate IslandThe secret room is under the detention room, and can be reached through the filing cabinet.Astro Knights IslandMordred's secret lab is underground, with a locked entrance under the hay bales.(see related questions)
The blueprints to the Secret Room in the school are inside Nate's overstuffed locker. Use the combination (9305) that you find on the completed comic strip. To enter the secret room from Detention, you need the blueprint and the Stink Bomb, and, of course, the gum that gets you into Detention.
let the stink bomb go
It it in the secret passage in the detention room on Big Nate Island.
in the detention room
In the drawer cabinet in the detention room.
Go inside the file cabinet, go down to the secret room, and get the bell clapper. Before you go to detention, you have to open Nate's locker and get the blueprint showing the secret room.
In the file cabinet, in detention, in the school.
You are only in detention to get into the filing cabinet, which has a secret passage behind it. Before chewing gum to get detention, there are two things you need : a stink bomb and the blueprint from Nate's locker. (see related question)
The teacher (Mrs. Hargrove) is in the detention room while you are hunting for the Time Capsule. When you find all the comic pieces, you can enter the secret room under her detention room. You will need the gum to chew (to get detention) and a stink bomb from the Science Room (mix yellow and blue, heat them). (see related question)