This is a One-way Door?

I came to school this morning and noticed that I had forgotten my sweater. The classroom is like an icebox - in Brazil, it seems, it's always hot outside and always cold inside - so I always wear it.

So I went down to the ground floor with the intention of getting it. According to my clock, it was 7:05. I had twenty minutes. Twenty minutes to cross the street, go to the apartment, get the sweater, and come back. Piece of cake.

I was walking quickly and I shouted to the doorman that I was just leaving to get my sweater. And I made for the doorway.

He told me I couldn't leave that way.

What? I thought. What kind of nonsense is this? A one-way door?

Basically how I felt.

He explained that you could only come into the building through this door until it was time to leave, at which point you obviously exited. But if I wanted to leave now, in the morning, before class, I had to use another exit.

Oh. Right. Because that totally makes a lot of sense. Does it matter how I exit, as long as I do so without causing a disturbance? I really hope there's some logical reason behind this.

So, I headed to the other side of the school, and I checked all of the exits that I knew of. And, lo behold, they were locked too! The doorman most likely knew this too, duh. I probably just missed one.

More annoyed than anything else, I made my way back to the classroom. It was still ten minutes before anything was scheduled to start - a lot of kids hadn't even arrived yet. I've left the apartment five minutes before class was due to start and still made it on time.

From Turkey. Also not understanding something.

The reason I'm telling this story is because I don't understand why I wasn't allowed to leave. He knew my intentions. If I wanted to cut class, I wouldn't have come to school, and I certainly would not have left my belongings in the classroom. I would be back before class started. Almost every day, he sees me go to school either wearing that sweater or with it in my arms.

So why is this an issue?

Perhaps it's just a quirk. It might just be the simple reason that once you're in the building, you're not allowed to leave, for whatever reason, until the end of the day. But I don't understand this either, since if I wanted to skip class, I wouldn't have come.

Not understanding. That seems to be the order of the day.