The premise of the doujin I'm looking for was a lolicon femdom teacher-student relationship. theres a scene where the student acts like she's gonna cut the teachers dick with a box knife but she touches it with the blunt side to mess with him. later on there's another student having sex with the teacher and it evolves into a bdsm love triangle. i remember the first one had black hair and the second one was an ppai-loli with glasses and short hair. there's also a scene where the first student explains that a teacher from her last school tried to rape her and she either killed or hurt him badly.