For reference, names of side Characters:
- Senka (センカ) - Grey-haired swordswoman, stays by the principal.
- Yoko (ヨーコ), the brown gal investigator girl.
I believe I got:
- END1: Win all battles and don't visit the principal much. After defeating the delinquents, the principal tries to brainwish Ouka and Yoko, but Senka saves the day. Happy ending that unlocks all HCGs.
- END2: Lose first battle, win second one. The scene goes to the principal who brainwashes Ouka and makes her pregnant on the next school orientation.
- END4: Win all battles, but visited the principal too many times and caused Senka to be fully brainwash, which means she won't save you at the end.
- END7: Win all but the final battle. All the girls are violated outside front of school.
- END8: Lose all battles and see all side character events (including Senka events). School gets rule by delinquents and Ouka goes to other school delinquents to prostitute herself. For some reason, this ending screws up the final save by locking the final door. Probably something about chapter numbering from 4 to 5 and the fact that viewing the ending auto-saves to keep the status before returning to the title screen.
My guess, but there is probably an ending for not watching Senka events but still losing every battle, or possibly any loss during the winning routes.
And the guy who appears at the back of the gym after some time, not sure what he's asking for either.
He helps you play videos of background scenes for Yoko during the losing routes. There should be two of the videos that you can pick up in the 3rd room of the delinquent's hideout (marked what seems to be the girl's bathroom sign and has those wooden triangles), one in each chapter. After viewing these and some more scenes, you can find Yoko in the upper-left side of the park for her final scenes.