Hi,

I'm looking for this manga.

It is set in a post-apocalypse Japan, there are three or four girls and a boy/man. They have to survive and there are these strange monsters that appear sometimes.
There are also strange events, which I can't describe unfortunately.

I'm not talking about Gakkou Gurashi.

If you have any idea, please answer.

Thanks! 😀

Sounds very much like "Earth's After School Rest" aka Chikyuu no Houkago

http://www.mangaupdates.com/series.html?id=41344

Bang on the money. Thank you very much!

You're welcome. I love this series!