Sotsugyou M is a video game and also a manga serial comprising of five volumes which were authored by Arisugawa Kei and sketched by Sugisaki Yukiru. They plot the adventures of five boarding school students in their final year as they prepare for their "sotsugyou" or Graduation. There are also four other manga volumes (Sotsugyou M PULASU (Plus) 1-4) who were sketched by a different manga-ka, Nishiomi Kyouko. They are also twinned in a character/music sense with the seiyuu unit E.M.U (1995-2000) as each of the five Seiyuu in the singing unit represents one of the five "M" characters. More about E.M.U as a musical outfit here.

It's no accident that E.M.U's band name is pronounced 'ehmu', which is the Japanese pronunciation for the letter M. M is, in fact, the nickname for the quintet of boys.

N.B:
Although EMU were the real life equivalent of "M", the boys aren't actually a band as such for the most part. In fact, they only become a band at all in one of the drama CDs (Mou Ichido Labyrinth) and in the final volume of the PLUS manga. This means that the band theme was added into the plot later, probably to complement the E.M.U unit's existence. It's also worth noting that in the band E.M.U, Hayashi/Kanna Nobutoshi plays guitar, and in the drama CD, so does Nakamoto. However, in the manga, Nakamoto plays keyboard and Mikimaro plays guitar, probably because it's a more logical instrument for Nakamoto to play given his existing ability on the piano.

Since it's often referred to as "Male Graduation M", its probably logical to interpret the "M" as standing for "Male", or equally, it might stand for "Minna", the Japanese word for "everyone". But that's conjecture on my part. The five characters are Takagi Shimon, Arai Tougo, Katou Yuusuke, Nakamoto Shou and Shimura Mikimaro. More about them here.

There are also a series of  novels (at least eight or nine, I think, but all out of print and I haven't tracked them all down yet), Drama CDs, 'live radio' CDs, interview CDs and basically a whole array of M media, bits and pieces.

 These drama CDs and interviews are largely the connection between Seiyuu and Characters - the voices behind the faces, as it were. More about the Drama CDs can be found here.

In terms of the animated side of things, there has only been one venture for Sotsugyou M and it came in the form of a two part OVA titled Ore-tachi no Carnival. The EMU boys here appeared once more to voice their characters - more about that here.

Additionally, and further forging the connection between characters and Seiyuu, the E.M.U first live concert featured a live Sotsugyou M drama when the boys performed as their characters on stage.

E.M.U's musical releases came in two forms - as E.M.U and as the characters from M. For this reason there are releases as Sotsugyou M and releases as E.M.U. This distinction is especially noticeable when you consider that the final E.M.U release - the Memorial box set - included pretty much every E.M.U release (and some new studio recordings), but did not include any of the Sotsugyou character songs. Equally on the disks for Sotsugyou M, the seiyuu are credited as their characters as well as as themselves. Of course, although the early album ("Girl") songs are not credited to the M characters, most of them have distinct M tie-ins - for example the solos are generally related to the character of the singer, and songs such as Boku-tachi no Kotae, Shounen A-Z etc are closely linked to M through the manga and through the MVP releases (including spoken lines from the M characters at points). Many of these songs also provide background music as instrumentals on the various Drama CDs. These songs DO appear on the Memorial set despite that, as does the final E.M.U song which is actually credited to the M characters (Oretachi No Sotsugyou). The lyrics to Oretachi no Sotsugyou are printed in the final scenes of the final volume of the PLUS manga, actually played by "M" to mark their graduation.

So the distinction is far from clear-cut. Confused yet? Yep. It's a bit like that :)

For each of the characters, a mini-disc single release was also produced, featuring one song and one short mini-drama monologue by each seiyuu. There is also a group release featuring the song 'Last Dance' sung by all five of them, making six disks in all. These disks, known as "Character Special" disks, are among the most difficult Sotsugyou M music memorabilia to find. More about Sotsugyou M's music releases here.

Equally - and quite bizarrely - there was a release of "Voice Message" disks for each character. These featured interesting and amusing things such as answerphone messages, wake-up calls, congratulatory messages etc and also came in the form of mini-disks. These too are considerably difficult to find on the second hand market, even if you look on Japanese sites.