First, go upstairs to the room with the trench gun.

Then, find the question mark on the floor.

Then, sprint towards the broken wall next to the question mark.

While sprinting jump and then hold B while jumping into the wall.

Now, zombies are blocked from you. You can tell its worked if your body is half through the wall.

Beware though, the zombies can push themselves though if there are a lot of them!