Its a short story about a nymphomaniac female teacher who films group sex with strangers. At work she is blackmailed by students to have sex with them but that part ends up only being only her imagination.

Another one I forgot the name is about a collage student who lives with a elderly couple and has a affair with the husband.

I am pretty sure the first one you are describing is called 'Empty' and it's the eighth chapter in this book, No Mercy (INOUE Kiyoshirou) because I cannot imagine a prostitute teacher getting blackmailed by students and then it all being in her head is a common plot.

No idea on the second one, sorry.

[img]http://kofk.de/C2C51774.png[/img]