Why was there a locked gate inside the school?

I'm watching Breakfast Club and I can't figure it out. Why, inside a completely enclosed school, would there be a locked gate?

we had giant doors that locked off sections of the hallways in my school. i think it was to keep out people who were there for games or the annual festival from wondering around by the classrooms and offices.

We also had gates at various sections of my school for the same reason.

yes. same here.

We had locked gates with "interesting stuff" behind (acetylene torches; concentrated acids; swimming pool).

To keep the zombies back:

Duh...it's the special leper wing of Shermer High.