A sandbox dating sim game made by the Japanese company ILLUSION.
In it, the player creates a 25 character classroom of students, and plays as one throughout a school year. The game is free of any real game "mechanics", minigames, schedules and does not even have an ending (except for the bad ends wherein the character dies or is expelled). The appeal comes from a the cast of characters and from the extensive customization options, which can be furthered by the use of mods.
The game has never been translated into any language and has not been legitimately released outside of Japan, but scripters and modders on hongfire, 4chan and other sites banded together to create an extensive "release" for the game, containing a huge amount of modifications, modding options, custom characters and voices.
You can buy the base game (in Japanese) from ebay with no difficulty, but it's region locked so you won't be able to play it.
If you're interested, users on 4chan have done an admirable job making it available, if not easy to obtain. An English-language install is possible, but you will need, among other things, the base game (off a torrent) an emulated CD mounter/burner such as DAEMON tools, the Illusion Wizzard by DarkHound, the dialogue translation, the HGG UI translation, the translated names pack, all the DLC (also from a torrent) applocale for reading the Japanese-language files (if your system is running 8.1 on windows or lower) or a locale emulator (if using windows 10). The game cannot be run on mac. You'll also need the install guide found on the aa2 general http://boards.4chan.org/vg/catalog (ctrl-f "aa2", you'll see it) as the install process is exceptionally complicated.
