I poked around a little. There's some information on these sites:
http://www.ne.jp/asahi/4-leaf/kmr/gm/gmetcindex.htm
http://www6.ocn.ne.jp/~diatonic/cd2_right.html
as well as an expired YJA auction with good scans of Mystery Case in Hi!! School!! There is a cat number on the inside scan, but I can't make it out.
http://page5.auctions.yahoo.co.jp/jp/auction/e77994923
Plus a couple of albums for sale:
http://page5.auctions.yahoo.co.jp/jp/auction/e76736620
http://page.auctions.yahoo.co.jp/jp/auction/99941255
Anyway, I put together everything I found on Mystery case in Hi!! School!! and put up a page:
http://www.vgmdb.net/db/albums.php?id=8445