Thank you.
For the bug, probably you are right but I still remember the time I really can't get the last note in the meeting room to get ending 5. I directly went to meeting room several time. However I just tried again ,the door to meeting indeed lead to the correct place. Anyways the note doesn't affect the gameplay .I found lots of little bugs in the game play ,this is just one of them. I don't think it's a big deal.
More bug example if you want: hiding in toilet and get stuck. Also, directly go to the principle's room ,Yuri comes ,use the bug to music room ,get the key in professors' lounge ,then go back to principle's room and cannot get in.
But for the secret ending, I still don't have any idea. My route is first go to4/F and get the mysterious letter ,then go to Yuri's room and sex scene ,choose yes, then no. After this, the main character will "grab" the key and leave. However I notice that there the key is not in backpack. I followed your instruction ,back to Yuri's room, back to normal school, then go to Angel's room to get the key but when i come back, Yuri is not in her room. I am confused about this part. I later played the game normally but I only get ending 3 in the basement.