the previous person will say what has he/she lost at Hogwarts, then the next person will say when and where did he/she find it, then say another one.

eg.
1: I lost a piece of parchment with Rune characters, it was my SoAR homework.
2: I found it in the library, on a desk next to the window. I lost a quill, it was a plain one. It's just a white feather. It has been used for a long time. (suppose he is a Ravenclaw)
3: found it! it was in the Ravenclaw common room, on the sofa. Did you fell asleep holding it? I lost a key ring ....

I'll start first.
I lost a bottle of blue ink. It was the colour of the starry sky

I found it in the Abandoned Toilets
I lost my wizard chess set. It's just an ordinary one.

i found it in the owlery! the owls were playing chess with it!

i lost the notes i took during transfiguration today

I found them in the toilets as well- crazy the things that end up in there. It was even in a toilet bowl!

I lost my book on mythology

found it on the library desk next to the windows! must have left it there after reading!

I lost my necklace with blue sapphires

I found it in the Giant Squid's mouth! Funny how it ended up there.

I lost my sanity. You can't see it but I used to have it

I believe I found it in the Hufflepuff common room, it was next to the paper cabinet.

I lost my friend who keeps wanting to commit arson. Not a very good thing.

found sitting at the edge of the forbidden forest, why there?

lost my parchment full of charms notes

I found it in the North Tower!

I lost my galleon! I last saw it under my bed don't question