PC-based hentai adventure, released by Elf Games.
Isaku is a tale of a group of students (and their teacher) trapped in their school while a homicidal gym teacher attempts to capture and rape them.
You play Kenta, one of the students. The rest of the cast are made up of four female students, Kenta's two male friends, and Kumiko, the female teacher. Obviously, with this being a hentai game, the teacher is hot and the girls fall into the standard anime chick categories (the young naive one, the brainy one, the ditzy blonde one and the friendship-blossoms-into-love one).
The game is entirely in Japanese, and since I can't read Kana, I can only go by what my FAQ tells me, which is that you spend the whole game looking for a way out of the school. It turns out that only Isaku himself (the gym teacher) has the keys, and therefore to win the game, you must not only prevent him from capturing any of the girls, but also kill him.
Each girl (and Kimiko) have a hentai scene when they are captured by Isaku, which you see during the game (and thus means that even if you finish the game, you get the 'bad' ending), but in an interesting twist from usual H Game law, you don't automatically get the pr0n accessible from the main menu - you have to find the 'videotapes' that Isaku made of the sex.
Although Isaku is an enjoyable game to play (even for non-Japanese speakers), the content is fairly extreme, and may not be to all tastes. However, the artwork throughout is excellent (amongst the best of any manga-style game I have seen), and the plot is not as linear as many of the genre. An English translation would put it in the top five gal games of all time, but for now it remains something of an undiscovered gem.