@Rin2k, Yes i read Hammer Session, and in that the Teacher's backstory did get explained to us in the few first chapters, proving my point.
Like i said, this manga is the first in GTO-genre that did it in quite a refreshingly different that your usual GTO-like stories. Like for example: Even up to this chapter we haven't really seen Jimi-sensei in his off-hours, compared to GTO and HS.
It's fine if you find the usual GTO-formula more preferable than this but i am personally happy that this manga is not just a re-skin of GTO or HS.