In the Detention block, more towards CP 3 than 2, enter the hallway that does not directly connect 3 and 2, and, from CP 3's side, face the right corner. Look at the blinking panel on the corner, how it slightly floats off the wall. Attempt to walk at that corner and, with a bit of readjustment, you will walk through the wall. However, there's only a thin patch of platform there, so you're most likelygoing to fall to your doom. With skill, you can walk along the entire corridor like that, being able to shoot inside without being shot back.