It is behind the filing cabinet. You must open Nate's locker on the second level. Pick up the blueprint from the pile in front of it.
(see related question)
The entrance to the secret room under the detention room is in the filing cabinet.
in the detention room
It it in the secret passage in the detention room on Big Nate Island.
It is in the secret store room behind the filing cabinet in the detention room on Big Nate Island. You need the blue prints from Nate's locker in order to access the secret room.
In the file cabinet, in detention, in the school.
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.
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.
The stink bomb is used to get the teacher to leave the Detention Room. When you have the blueprint to the secret room (from Nate's locker), use the gum to get detention, then run the teacher off with the stink bomb so you can get to the file cabinet and then the secret room below. (see related question)
When You Go Into The Secret Detention Room And you jump down the Light switch is on the wall.
in the detention room.
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)
Among the tons of junk crammed into the locker is the blueprint to the school. It shows the secret room behind the filing cabinet in the detention room. (see related question)