When?

I stood outside the big glass compartment and wondered why I was summoned there.
There were many seats on the other side, a table stood on the far end towards the left and there was a whiteboard hanging on air.
It was a classroom set up.
I could not get in. The glass acted as a barrier and I could not find any door to let me in.
A fog suddenly enveloped the 'classroom' on the other side and my eyes started to smart. I blinked and rubbed my eyes, wondering how I could be affected by the fog when I was on this side where the air was clear.
I looked inside the 'classroom' again and realised that the fog had cleared.
The class was filling up.
A man stood on the front of the classroom and he was saying something I could not hear.
Some people were seated while some others were walking in from nowhere taking their places. I looked around again and could not see any door for me to enter. I touched the glass and moved my hands around just so that I could feel for something that would help me get into the classroom. I started walking around but then, I realised that as I moved, the 'classroom' moved with me.
I stopped and looked hard. A familiar face was seated at the far end of the 'classroom'.
I gasped and choked!
It was my late sister.
I started knocking on the glass that separated us furiously.
I had to get into the 'classroom'.
I must see her!
I called her name but no sound came out from my throat. I ran towards the front just so that I could get some attention from the man at the front of the 'classroom' as well as some of the students in front.
No one was paying any attention to me.
I attempted shouting and again, I could hear no voice.
I banged and kicked the glass.
I started crying. I had to go in and get my sister.
I banged hard and even hit my head on the glass.
There was no use. No one heard me.
I felt so desperate and fell to the floor.
I sobbed hard and my body shook.
I rested my whole body weight onto the glass.
My hand were splayed out wishing that the glass would just melt away and someone would let me into the 'classroom'.
Even in that pathetic position, no one saw me.
My sister did not see me.
She was watching the man at the front of the classroom.
I called her name out loud. I thought it was loud but even I could not hear myself.
I used my fist and banged the glass harder.
There was no use.
I was, after all, on the other side of the world.
I stood up, rested my cheek on the glass and called out my sister's name.
Like a miracle, the man at the head of the 'classroom' turned and stared hard at me.
I mouthed the word 'please...'
He turned and looked at my sister.
Like magic, my sister turned her head and got up from where she was seated.
She walked towards me.
She had a serene and peaceful look on her face.
She stopped as soon as she reached the glass that separated us.
I pushed my whole bodyweight onto the glass hoping that it would break so that I could just hug her.
She smiled and placed a hand onto the glass.
Then I heard her speak.
It was so clear.
'Do not worry, I am fine.I love you.'
Then, she turned away, leaving me sobbing uncontrollably.
She did not turn back to look at me.
The fog reappeared and when it finally cleared again, there was just an empty space on the other side.
I cried and cried.

It's been 11 years since my sister passed away.
They said time heals all pain but till today, I am still waiting for that time to come.
I miss her so much.