What was it??

I just want to ask this question..

Let me first give some context, i am a male and one day our teacher made all boys go out of the class ( it was 5/6 th class i guess) and close the door so that only girls remained in the class with door shut.. all the boys were wondering what the teacher was doing ?? This happened 2 times i guess..

When I asked the question to the madam I was slapped and was told that it wasn't related to my business.. and even asked one of my female classmate but she refused to say something..

So till this day I am curious what really happened that day??

And does this scenario even happen to others too..??