But that's still weird! How could they talk on the phone so calmly when you might spot them at any second?

That's exactly what leads... to our first conclusion.

The culprit's voice was so calm under pressure, it's almost unnatural.

That brings us to another important contradiction. Even if the culprit was hiding in the boiler room, the fact that the exit was locked means they must've snuck in from somewhere else.

If they'd used that hallway, the door would've been unlocked.

This means they should've had no chance to see the notice board. So how did they know the new number?

They can't have visited the high-security second floor personally, so they must've seen the notice board from somewhere.

From the lecture room? Impossible. We were having a lecture the whole time that the message was showing.

In the courtyard? No, the kappa doing maintenance was there.

So......

Notice board

Window

Culprit

Courtyard

What about the hallway by the storeroom? You can still read the notice board from behind, right? Even if it's in reverse.

Good point.