Knocking from an Empty Locked Classroom

I have a story from my school days that's stupidly strange, and even back then it was hard to believe it was real.
The corridor with the classrooms was shaped like the letter "G", where the wing was slightly separated by an arch and sort of stood apart, and that's where our classroom was.
Some time before the first lesson, our teacher got sick, we were waiting for a substitute. There were only a few of us, about five people, sitting a short distance (about three meters) from the door, which was closed.
I hear the sound of a rattling handle. That distinctive kind. Well, fuck it, the other classrooms are open, maybe someone's opening one there. One, two, three, four times the sound repeats with some regularity. I ask the classmate sitting next to me if she hears it. She does, and another person nearby confirms it. Five, six. On the seventh time I lose my patience and egg on a friend of mine back then to go up to our door and knock, in case the sound is coming from it. We drum on it together, shouting something like "Hey dude, if you're in there — answer!" Less than a second passes, and they drum back at us from the other side. The door literally bends under the blows (anyone who's seen those new plastic school doors knows what I mean). I, my friend, and a couple of other people who were standing nearby saw and heard it. We ran to the front desk for the keys, sincerely believing someone had been locked in there. On the way we run into a teacher, we ask her to hurry, explain that someone's in there.
As you can imagine, when they opened the classroom, there was no one there. The windows were closed, the teacher looks at us like we're idiots. And no one except me really remembers this incident. When asked, they agree that yes, it happened, but nothing more.
And I can't help but remember it, because I still haven't found an explanation for myself.